I went from 5DIII to Fuji X-T2 recently; drivers were multiple: I was frankly annoyed with how much Canon DSLRs depreciated and how conservatively they evolve their product. The difference to change to a 5D4 from a 5D3 was £2400. I was fed up with the bulk of the 5DIII and 2.8 lenses - the camera I didn't carry because of its size led to underutilisation. I did consider changing to f4 lenses. That would have gained me (some) size advantage (not much). I looked at Sony - body size advantages, lovely IQ, but FF lenses were equally as bulky and big. I settled on Fuji because the X-T2 sensor offered some advantages over the 5D4, cost less than I'd lost on my 5DIII, and the lens system being built for a smaller sensor was smaller, and was sufficiently comprehensive for all my needs.
So far, I like the change. It's by no means perfect, in spite of the hype you read. I think the Canon handled nicer in some respects, controls, control positions, menus. Or maybe I'd just got to know my away around it. The advantages of mirrorless are real - the ability to properly preview DOF on screen, and histogram etc, before you take the shot, is ideal. The screen isn't quite wysiwig though - they have improved tremendously and lag is next to non-existent, but you are in no doubt looking at a screen and not at the quality of your final image. But then, you are not looking at anything other than a compositional representation of your final image in a mirrored camera. So it's a big step on. Battery life - not as good, but not an issue in my view, it might be for some. IQ - I feel it's better than the 5DIII in many respects. DR, noise in the shadows, resolution. I think the 5DIII probably bests it at extreme ISOs, at normal ranges up to 6400, I think the Fuji is better through to no worse, and that's in spite of a smaller sensor. I don't think the X-T2 is quite as quick to turn on, but it's no laggard. And the rear screen is smaller and the EVF doesn't feel as big as on the 5DIII. I love the Fuji lenses. While they are expensive, compared to non-L lenses, I feel I would have to buy L primes to match them, and then you are in a different cost and size bracket.
I'd say, figure out your drivers, where you are prepared to compromise and where you want benefits. I feel I have an equally capable camera, with improved IQ, albeit I'm sure the 5D4 would improve on it further in some respects, some nice new technology, and some lovely lenses covering an increased focal range at larger aperture than I had before and quite a lot of money in the bank. And the body cost me less than I'd lose if I stuck with a Canon upgrade path, even if the Fuji were worth Zero in 3 years time. I could only have justified the Canon if I were using it to earn a living and could write it down as an operating cost. As a value, enthusiasts system, the Fuji made a lot of sense to me. Have I stopped looking at the 5DIV - nope. But I doubt I'll buy one, as enough of my itches are scratched with the Fuji, and it's great fun to use.
Oh, and it fits nicely and lightly inside a Hadley Pro, instead of the heavy rucksack I used to have to carry. That was a big win. Whether it will fit quite so well if I indulge in the lovely 2.8 Zooms in the Fuji remains to be seen. They'll be lighter than a 2.8 canon setup, but I'm trying not to go down that route, as I'll be similar to an F4 Canon setup in bulk then. Albeit, I'd still have the option to throw on a small prime and have portability again. Best of both worlds maybe.
E-M5II compared to OM2sp 2 by Alf Branch, on Flickr
E-M5II compared to OM2sp 1 by Alf Branch, on Flickr
