Between camera bodies in the various formats, the differences are not in magnitudes. FFs tend to be heavier and 4/3rds tends to be lighter, whether mirrorless or not. Mirrorless tends to be lighter than mirrored. The upper end Pro Olympus E-M1X will probably be heavier than an entry level Canon or Nikon. But then one is comparing a pro level body to an entry level body.

The most weight savings will be seen in the lenses. I have previous to this particular thread entry compared 300mm Olympus to the Canon and Nikon 600mm (same angle of view). But compare the Olympus E-M1X, 2X teleconverter, and the new 150-400 f4.5 Pro IS 1.25X lens to the same Canon and Nikon 600. The 150-400 is 12.3" X 4.5" dia., 4.13 lbs., and $7499 (smaller, lighter, and cheaper than either 600). The range in 35mm terms, using the 2X to extend the range, is 300mm to 2000mm. The total weight of the E-M1X, 2X teleconverter, and the 150-400 f4.5 Pro IS 1.25X lens is 6.66 lbs. which is basically the same weight of the Canon or Nikon 600mm lens by itself while the Olympus pro body, teleconverter, and pro lens combination is still cheaper than either lens. Yes, that combination is still not cheap at $10.9K, but that is the most expense combination for any 4/3rds combination. Everything else is cheaper than that combination. And try to find a similar combination that will reach 300mm to 2000mm in FF or APS-C for $10.9K and offer 6 to 8 stops of IS. They don't exist and possibly never will. That is the true draw of 4/3rds.

Starting the 27th, Olympus will be offering sales on some of their better body / lens combinations that will start at $500. I am sure that Panasonic, Sony, Canon, Nikon, and others are going to have very good combination sales that will be worth looking into also.

I do not believe either Fuji or Olympus lenses are as good as Canon or Nikon as the two companies make the best lenses in my opinion, I used to use a Canon DSLR and a full set of Canon lenses many of them the white pro lenses
I changed my DSLR for weight reasons and bought a Canon m 50 with a am to e.g. Canon adaptor so I still use my e.g. Lenses now a days with a monopod.

I do not not know Nikons line up that well but would a similar body to the one I changed to suit and you could use your Nikon lenses perhaps with an adaptor and also use a mobile to help with the weight no doubt Nikon do a similar camera to my m50

SYSTEM weight (defined as the weight of the camera bag you carry around) will not change significantly if you keep using full frame lenses. For most of us, lenses are where the weight is. Oh, you can settle for smaller maximum apertures and save a bit of weight, but if you like to work in low light, that's annoying.

I have used many cameras, both personally and professionally. Today's cameras are incredible — period. I would rent to try before you buy. Basically, menus, ergonomics, weight, and feature sets matter a hell of a lot more than brand. All of the majors are making great bodies and lenses now. And since most of the mirrorless lens designs are less than ten years old, they benefit from the current state of the art.

That said, life is full of little trade-offs. EVERY camera is some sort of compromise. You give up some attributes to get those which are mutually exclusive. We can't buck the laws of physics.

My personal system is Micro 4/3. I use a Lumix GH4 for both video and stills. I have three pro-grade stabilized lenses — 12-35mm f/2.8, 30mm f/2.8 Macro, and 35-100mm f/2.8 (equivalent to full frame 24-70, 60, and 70-200 mm range), plus a couple of f/1.7 primes (15mm and 42.5mm). My entire system will fit under an airline seat. My bag is half the weight of what it was in my APS-C dSLR days, and 1/3 the weight of my kit in the film days of my youth.

I can, and have, written 7-page explanations of why I use what I use. It DOES NOT work for everyone, but it is exactly what I need. The best advice I can give you is go to http://www.dpreview.com and read reviews of the latest gear in your budget range. Then rent, and give the rental gear a good workout.

I would carefully define what it is you want to photograph. Define a kit weight you can tolerate. Define what is most important to you about a system. Define a budget. Then look for gear that meets your needs as best as you can find. Every body and lens has its niche application.

If it's Micro 4/3 you want, the Lumix G9 and GH5 are top-tier machines. Both are particularly good at BOTH stills and video. The G9 is the better stills camera, and the GH5 is the better video camera.

The Olympus OM-D E-M1 Mark II and the OM-D E-M1X are also top-tier. Others here have sung their praises.

If you like APS-C, the Fujifilm XT-4 is my target pick because their lenses are specifically designed for THAT FORMAT. Its film simulations in JPEG mode are great.

I'm not going to get into Sony, Canon, and Nikon offerings. Others will tell you their stories. Suffice it to say their mirrorless offerings were not there when I was in need of a new system. If a full frame mirrorless system were available in 2014, I would have bought it, so long as the audio features of the video section had pro features.