Hi Marianne,
You certainly have some good lenses to work with and, while it was a great little camera in its day, that T3i is getting pretty old. Maybe it's time for an update.
You are actually posing two questions. One is if you should "upgrade" from a crop sensor camera to a full frame camera. The other question is whether you should continue using a DSLR or "upgrade" to mirrorless.
The first thing you should ask yourself is whether or not you actually need a full frame camera...
Full frame versus APS-C
What do you do with your images? Unless you make really large prints... bigger than 12x18 or 13x19... you're unlikely to see much or any difference from the larger sensor format.
Another reason someone might want full frame is if they shoot a lot of low light conditions, requiring very high ISOs. Full frame cameras have less digital noise, so can typically be used at one or two stops higher ISOs. But, frankly, newer APS-C cameras have improved a lot handling high ISO too.
How big a part of your shooting are the birds and animals? Is your grandson involved in sports? If you are using the 70-200mm to shoot those, it will be much better used on an APS-C camera, than on full frame. To have the equivalent of the 70-200mm on your current camera now on a full frame will mean at least adding a 1.4X teleconverter or getting a longer lens like one of the 100-400s... or both.
However, a 70-200mm really isn't a whole lot of lens to use for birds, animals and sports.... even on a crop sensor camera that "acts like" a free 1.6X teleconverter. You might want to consider the Canon EF 100-400mm f/4.5-5.6L IS USM II lens instead. It's just a slightly larger and four or five ounces heavier than your 70-200mm. It would be a very good lens for birds, animals and sports on either crop or full frame... but for telephoto work, the crop sensor camera has the edge! It's like having a "free" 1.6X teleconverter, without the loss of light that an actual teleconverter would cause. The EF 100-400mm on an APS-C camera like your T3i or a newer model will "act like" a 160-640mm would on the full frame 5DIV.
Shooting insects with that 100mm macro lens, an APS-C format camera makes it "act like" 160mm would on full frame. With a 5DIV you may want to trade it in for a 150mm or 180mm macro. That will be bigger and heavier.
You mention shooting scenery, for which I presume you use the 18-55mm lens or the 24-105mm. For that purpose, you could do with an upgrade lens either way. If you stick with an APS-C camera, I'd recommend the Canon EF-S 10-22mm USM, which is currently hugely discounted right now to $399 ($250 off list price... Lens hood is sold separately, so add $30 to the cost for that.) With the 5DIV, I'd recommend the EF 16-35mm f/4L IS USM, currently on sale for $999 ($100 off regular price, lens hood included), an excellent lens without the larger size, greater weight and much higher cost of the EF 16-35mm f/2.8L USM III. For wide angle work, full frame cameras have the edge.
I would suggest you at least take a look at the new APS-C format Canon 90D. With 32.5MP it's got higher resolution than the 30MP 5DIV. The 90D has a high performance 45-point AF system that can do much of what the 5DIV's 61-point AF can do. In fact, all 45 of the 90D's AF points are the higher performance "dual axis" type, while only 41 of the 5DIV's AF points are. 90D can shoot at 10 frames per second, versus 7 fps with the 5DIV. The 90D also can buffer more images in a burst, 27 RAW vs 19 with the 5DIV. They both have 100% viewfinders that use true pentaprisms (bigger and brighter than your T3i's viewfinder with it's penta-mirror). They both have a native ISO range of 100-25600 (5DIV's is more expandable, though personally I never use expanded ISOs... too noisy). They both can shoot 4K video.... might be easier with the 90D's articulated LCD monitor (which also may be handy for macro shooting or scenic shots, if you use the one on your T3i for those). The 5DIV's LCD monitor is fixed... not articulated. But both of the newer cameras have a larger, brighter, higher resolution screen than your T3i. The 5DIV has dual memory card slots (1 SD, 1 CF), the 90D only has a single SD slot (same as your T3i).
Maybe best of all, the three-years-newer 90D is less than half the price of the 5DIV, even after all the current discounts being offered. That would leave a lot of money for you to put toward lenses, if interested.
Mirrorless versus DSLR
Okay, next question is whether or not to abandon DSLRs and go with a mirrorless camera. For sake of discussion, let's stick with Canon models (Sony is an option, can use adapted Canon lenses although there reportedly is some loss of AF performance). The full frame, 30MP EOS R is a "mirrorless 5DIV", and then some. The APS-C 32.5MP EOS M6 Mark II is a "mirrorless 90D", for all practical purposes.
The bulk of interchangeable camera sales are still DSLRs, although mirrorless cameras are steadily increasing their share of the market.
There are advantages... and disadvantages... to both camera types.
Mirrorless cameras can be smaller and lighter. The primary difference in size and weight is seen with APS-C format (and the smaller Micro 4/3 format). While it's true that full frame mirrorless cameras are smaller than their DSLR cousins, the lenses for full frame mirrorless are generally no smaller than the lenses for full frame DSLRs. In fact, in some cases the lenses for FF mirrorless are actually larger and heavier than comparable DSLR lenses.
Mirrorless cameras have electronic viewfinders that have some neat tricks. They can display a preview of the camera's current exposure settings, along with other useful info. But they also draw a lot more power than the optical viewfinders in DSLRs. As a result of that.... plus because they often use smaller batteries due to their more compact size... most mirrorless get nowhere near as many shots per charge as a comparable DSLRs. For example, Canon EOS R, 5D Mark IV and 90D all use the same LP-E6N battery. The EOS R mirrorless is rated to get 370 shots per battery charge, while the 5DIV is rated to get 900 shots, and with its newer, more efficient processor the 90D is rated to get 1300 shots. This is despite the fact that the 90D is the only one of the three with a built-in flash, which usually greatly reduces a camera's shots-per-charge rating. For example, the 7D Mark II (with built-in flash and older generation dual processors) is only rated to get 670 shots-per-charge. The EOS M6 Mark II uses a smaller LP-E17 battery and is rated to get just over 300 shots-per-charge.
The above are just manufacturer ratings based upon a standardized testing procedure. With some reasonable care to conserve power, most people get a lot more shots-per-charge with all these cameras. But, even so, depending upon your usage, more spare batteries may be necessary with a mirrorless camera. Something that can improve matters is an accessory battery grip, which increases the size of the camera, but also doubles battery capacity and provides useful secondary controls when shooting with the camera in portrait orientation. Battery grips are available for EOS R, 5DIV and 90D. But none is available for the M6II.
Another possible concern with electronic viewfinders is "blackout"... This occurs with EVFs when making a continuous burst of shots at a high frame rate and can make difficult shooting active subjects like wildlife or sports. While manufacturers have reduced blackout to a large extent, the optical viewfinders used in DSLRs are still preferred by many people shooting those subjects.
In fact, the M6II doesn't even have a built-in viewfinder. But you don't have to rely upon only the rear monitor screen either. There's an accessory Canon EVF-DC2 viewfinder available for use with it. That fits into the camera's hot shoe (so auxiliary flash can't be used at the same time as the viewfinder).
Speaking of lenses (we actually weren't yet, were we). The system of lenses for Canon and Nikon DSLRs is much deeper and more comprehensive than for any mirrorless camera system. There are especially limited native lenses so far for the EOS R and M6 Mark II. Yes, you can adapt the DSLR lenses for use on the mirrorless cameras.... But by the time you add an adapter, there goes the size and weight savings out the window. Particularly with smaller lighter mirrorless like the M6II, some users find them a bit uncomfortable and unbalanced to use with larger lenses.
Canon is rapidly developing the RF-series of lenses for the EOS R. In a little over a year, they've introduced ten RF lenses and promise more this year. Most of those are very high end and cost more than comparable EF lenses for the DSLRs. So while the EOS R camera costs a significantly less than the 5DIV, a lot of those savings disappear when you start to buy RF lenses for use on the mirrorless camera.
As feverishly as Canon has been developing lenses for their R-series FF mirrorless, they've done the exact opposite with the their APS-C format mirrorless cameras. They introduced the first M-series almost eight years ago, but have only developed eight lenses for them... and most of them are more entry-level quality. For example, they all use Canon's STM focus drive, which is good but not as fast as the USM they use on their premium lenses.
Compare the eight Canon EF-M lenses for Canon M-series (APS-C) cameras and the ten current Canon RF lenses for Canon R-series mirrorless cameras.... to the approx. NINETY EF and EF-S lenses currently available for their DSLRs. In time, I'm sure we will see more... especially RF lenses. But it will be many years before the lenses for Canon mirrorless come anywhere close to the selection and versatility available for DSLRs. Compare to other systems that have seen serious commitment to lens development for longer period of time... Fuji and Sony have the most lenses for their mirrorless... and between them Panasonic and Olympus M 4/3 systems (which share the same mount) have a fairly large list... But Canon and Nikon DSLRs still have at least double the availability of any of those other systems.
In the end, I'd suggest you first decide whether or not you need full frame, then decide whether DSLR or mirrorless meets your needs best. You may have some important considerations I've not touched upon at all here. Of course there are other possibilities, but the four Canon I'd recommend you consider are...
EOS M6 Mark II, crop sensor, 32.5MP mirrorless.... $850.
EOS 90D, crop sensor, 32.5MP DSLR... $1200.
EOS R, full frame, 30MP mirrorless.... $1800.
EOS 5D Mark IV, full frame, 30MP DSLR... $2500.
You need to consider whether you'd see greater benefit from an additional or different lens, versus a different type or format camera. You may even need to consider changing to an entirely different brand, which should entail selling all you've got now and replacing it with more appropriate gear.... But you also have to look closely at those alternatives to see if they truly fit your current and future needs.
If you do a search for "full frame versus crop sensor" you'll find dozens of online discussions about the pluses and minuses of each format. And if you do a search for "mirrorless versus DSLR" interchangeable lens cameras, you will also find lots and lots of conversations. I'd recommend you spend some time reading those for more detail than is possible here on UHH or on any other forum. While reading those, try to keep in mind your own particular uses now and in the future. Hopefully that can guide you to the best choices for you. All too often, when you ask questions like yours on a forum like UHH, you'll get advice and opinions biased by the choices people have made to meet their purposes, which may be very different from yours.
Good luck and have fun shopping!
Hi Marianne, br br You certainly have some good... (
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