In advanced mirrorless, I thing the two choices are Sony or Fuji. I think the Fuji has it for customer service and build quality, but Sony has more glass available (but not necessarily better glass). The other idea is to wait to see what Nikon puts out in mirrorless. But O don't think you would regret getting the Fuji xt-20. I have the xt-2 and the 20 is about 85% identical. The lenses are beautiful, sharp and mechanically very smooth operating. The jpegs are gorgeous right from the camera, and all the controls are on the body and operation requires a minimum of menu diving.

Good info!

But I have to question if Sony actually has more lenses to choose among. Only looking the sheer numbers it would appear so.... 44 Fuji X-mount versus 63 Sony E-mount. However keep in mind that all the Fuji X-mount are designed for one format: APS-C.... While Sony makes lenses both for APS-C and full frame formats (19 APS-C, 44 full frame). Of course, I'm sure their APS-C cameras can use both, while their full frame cameras are limited to full frame lenses.

But the above numbers taken from a retailer's website are a bit misleading because both manufacturers make some lenses in multiple versions and have overlap in other respects. Fujifilm website lists a total of 26 X-mount lenses (15 primes, 11 zooms), plus two teleconverters. The Sony website actually lists 42 E-mount lenses (20 primes & 20 zooms in both formats), plus two teleconverters. But, for example, in the Sony system there are very similar 16-35mm full frame, 16-50mm APS-C, 18-55mm APS-C and 16-70mm APS-C lenses.... there are also three different 18-200mm (all APS-C).... and two different 24-70mm along with a 28-70mm (all full frame). There's some overlap in the Fuji system, too... But I don't think nearly as much as in the Sony (due in part to making lenses for two different sensor formats).

I'd recommend carefully comparing the two systems side-by-side.

Fuji has long made really top-quality glass for many different formats. Sony may be comparatively new to serious photography (having bought the Konica-Minolta system in 2006), but they have a relationship (?) with Zeiss, who also have a long history of quality lens making.

Just from comparing and considering the two systems (and others) for my own possible ventures unto mirrorless, it appears to me that Fuji gear might be a bit more expensive.... at least initially. Sony cameras and "kits" appear to cost less, possibly are "loss leaders". I.e., more affordable but less profitable, to get you to buy into the system... after which their lenses tend to be a bit pricey (more expensive than comparable Nikon or Canon DSLR lenses, for example).

The main complaint I've heard about Sony is that their menus are somewhat difficult to navigate and have to be accessed too often to handle various functions.

Just a note regarding the Sony/Fuji comparisons that is worth being aware of... Fuji does not manufacture & release new camera bodies nearly as quickly as Sony. Fuji's mantra is based on a solid product (like it or not) and constant firmware updates available free for users. A two year old XT2 will be updated several times before becoming "obsolete" making it a long-term relationship between user and factory. Not only is this philosophy welcomed, & sensible, but also a much cheaper experience for users. Cheers!

I have two Fuji X-T2's which replaced my Nikon D700 and D800. I don't do birds but I've seen great shots taken with the Fuji 100-400. I use the 50-140 f/2.8 for portraits. One of my bodies came with Fuji's 18-135 f/3.5-5.6 which I find very versatile to carry on trips abroad. I'm 80 and I can assure billnikon that we are not talking about "a few ounces" difference in weight, but a few pounds, to say nothing of bulk. I can curl a 30lb dumbbell 10x so I can carry a Speed Graphic if I was ridiculous enough to do so. Perhaps hauling around FF Nikons and their lenses while walking around Palermo or trekking through Borneo's jungle to photograph orangutans is billnikon's speed, but count me out. Sometimes I will go out with just my iPhone X to experience the liberation of not being bogged down by apparatus.

Interesting, you are not the first to profess your desire to go mirrorless simply because of WEIGHT. Interesting that a couple of ounces have convinced hundreds of DSLR photographers to switch.
I am currently 70 and have used SLR's professionally for years. I still use large DSLR's, D4s with a 200-400 lens. a D500 with battery grip (I like the weight) and the 200-500 lens attached.
Why are folks forking over thousands of dollars for a few ounces, is their another reason to switch besides WEIGHT. I get award winning photographs with my current equipment and have no problems with WEIGHT. I just don't understand.
I will be ordering soon the new Nikon 180-400, I am not a strong male, but I find WEIGHT not an issue.
Am I missing something? Should I start going to the Y more? Will I get progressively weaker quicker in my 70's than 60's? What?

Interesting, you are not the first to profess your desire to go mirrorless simply because of WEIGHT. Interesting that a couple of ounces have convinced hundreds of DSLR photographers to switch.
I am currently 70 and have used SLR's professionally for years. I still use large DSLR's, D4s with a 200-400 lens. a D500 with battery grip (I like the weight) and the 200-500 lens attached.
Why are folks forking over thousands of dollars for a few ounces, is their another reason to switch besides WEIGHT. I get award winning photographs with my current equipment and have no problems with WEIGHT. I just don't understand.
I will be ordering soon the new Nikon 180-400, I am not a strong male, but I find WEIGHT not an issue.
Am I missing something? Should I start going to the Y more? Will I get progressively weaker quicker in my 70's than 60's? What?

Drawing conclusions from a 4 year old YouTube test may be enough for you, but one credible claim that the Fuji X-T2 up to date with firmware and a better lens than the tester was using (the 50-140 f/2.8 is better) is faster than the Nikon D500 can be found here:

https://www.youtube.com/watch?v=zPnIKPwaGsA

See also: https://www.imaging-resource.com/PRODS/fuji-x-t2/fuji-x-t2-conclusion.htm

"Overall performance significantly amped from the X-T1
Mirrorless cameras have strived for the better part of a decade to catch up with competing DSLRs in the C-AF arena. Top-tier DSLRs still rule the professional sports market, but the lines continue to blur in the enthusiast world, and the X-T2 along with its rangefinder-inspired brother the X-Pro2 have helped to continue that trend. In our various testing we found the C-AF to be significantly better than the system found in the X-T1, and to now belong in the same ballpark as competing DSLRs in this enthusiast price range."