I just did a small test shooting with the Fuji X-T1 with the 56mm 1.2 and my old Nikon D90 with the 50 mm 1.8D. I was looking mostly for moving objects with some stationary thrown into the mix.
For the moving I tried both continuous and single-focus, both one shot or multiple at the max speed.
The area was tough; low light, requiring 1.8 at ISO 1600 or more.
I tried the 56 both at 1.2 and 1.8 to level the field.
Focus speed wise the D90 won clearly. Both continuous and single shot. Accuracy on D90 was good when it locked focus, slightly less than X-T1. The X-T1 almost felt like it has significant shutter lag, it might have been the focus release priority though. For single shot the X-T1 was also somehow more accurate.
The D90 would just lock focus and start on shooting (with variable degrees of accuracy) while the the X-T1 was taking its time and sometimes the subject was already gone; when it fired in continuous focus it wasn't always accurate because by the time it fired the subject was gone.
I was less interested in the technical quality but overall as the Fuji exposes more to the right and has really nice colors without much work the pictures from Fuji combination were nicer.
Anyway, I came to the conclusion that no matter how much I try the Fuji (at least the X-T1, maybe will be better for X-T2) cannot compete in focus speed and continuous focus with a modern DSLR, and as I will need this from time to time I am thinking about getting the D750. Better movies, DR and low light performance are bonus. Oh, and I don't like X-Trans, too much headache for unclear benefits.
While I have some ideas about the primes I would get (and I already have a 50 mm 1.8D and the 70-300 4.5-5.6 VR) I am still puzzled about a walkaround zoom.
The 24-120 F4 is appealing because of the range and weather sealing. However it is not that bright and only provides slightly better resolution vs my Fuji 18-55 2.8-4 despite 24 MP vs 16 MP, and the new 16-50 F2.8 might make this much less clear. It is 600$ off currently.
The Sigma 24-105 F4 doesn't look significantly better. The Nikon 24-85 has a great range and it's light but it is not bright and it's not sharper than any of the other options, particularly in the corners.
The 2.8 zooms are heavier than I would like and a little reach limited but if really needed I could get one. Here the Tamron 24-70 seems the best value, I particularly like to VR, but the Nikon version might be better on the other accounts. All in all not one of them looks like a really great lens that is clearly better than the others.
Focus speed is the main driver but I would like an upgrade on more than that.
Any personal experiences with the above?
PS. the D810 could still be an option but the D750 seems to be the better value by far and I like the tilting screen; 36 MP with no AA filter are difficult to ignore though. Most of the other extras the D810 have don't mean much to how I will be shooting. Not really looking at the D7100 as I want the step up in image quality and better lens availability if I'm going to do it.
PPS. the D90 with the 50mm 1.8D is easier to hold than the X-T1 with 56mm 1.2 even as the overall weights feel similar.