I never used the U1 U2 stuff. Can someone give me examples or common settings they have, so i can test it out, please?
You go into a shooting mode, make lots of changes, save them. with the U1 saved...you then reset the shooting mode. Do over over for U2 if desired.
It basically gives you several custom shooting presets. U1 and U2 and whatever is currently stored in the shooting modes. In my case I have U1 and U2 and whatever is programmed into aperture and shutter priority mode. Manual mode kind of set but I only use for video and most of those settings are video menu based. I am finding pure manual mode the most reliable way to keep shutter constant for video.
Currently I have U1 set for flash. Custom changes include timing changes, number of focus points available (limit it to least number, think 21...) and other things like AF assist turned off (otherwise known as the annoying light to me that shoots out before a shot). Other changes include ae lock being AF selector to get off the shutter button. For reasons unknown Nikon puts a dedicated af button on their pro bodies only. I liked this feature on my d700 (now use d750)...I got it back in a way but would be happier if had a real button instead.
Rest of this...while eyeing the higher iso potential of the d750 keep in mind even it can need good post processing software for noise reduction. This same software could serve your d90 shots. I settled on DxO. Used to be aperture with nik tools installed but that went away. tried going back to adobe with nik, realized why I left abobe in the first place...and found a new alternative in DxO. Try what's out there on free trials, software a personal choice as always.
How you hold the camera can also help here. Good solid hand hold and decent shots at iso 6400 are doable. Even with the d750's high iso I still try to keep her at iso 3200 or 6400 worst case.
Also remember the main thing higher end bodies offer is better shooting control via menu's. Not using these to potential on a d90 some of the oooh ahhhh effect will be lost on a higher end body. D90 a capable body, had one many years ago. I left as I wanted full frame.
Which ties into knowing why you are going fullframe to begin with. My case was I can go landscape heavy at times (FX liked for landsacape). I also like nikkor's older glass from the Ai(s) days (manual focus has its allure..and these can be picked up for decent prices nicely). They are FX glass. Zeiss makes some nice stuff too for modern manual focus glass.
Of late this was a side bene....manual focus glass a better fit to video work. Much smoother and better feeling focus ring adjusting usually. Dslr video the need to manual focus becomes clear fast. As even the most silent AF motor lens I have craps on the shoot's audio with a quickness.
Video being brought up as its the d750's other thing. big + 1 to why I got it. If getting for any video work...just order an external mic for it same day. Don't go I will just use the on body mic. You will learn the error of your ways...then have to wait for the new mic to come in when you order it lol. Been there....doh.