I've not used the N90, but I've heard one should avoid the non-s version, is there any merit to that?
Not exactly. The original F90/N90 is 95% or more of the updated F90X/N90S (sincerely I don't know why Nikon USA had to give it another name!)
The F90X is an update: the AF motor was beefed up, accuracy and speed improved. The shutter speeds can now be selected in 1/3 increments, the beep position on the ON/OFF switch disappears (can still be selected using the PC software), the data memory (only usable with the PC connection) was enlarged, the 2 shots self-timer setting is gone and the motor drive speed is improved. One other change, and important for outdoor photography, is the improved weather seals. And finally, the power grip was added to the accessory list. The F90 can use the MB-10, but the vertical shutter button can't be used as the F90 lacks the contacts for it.
There are other minor changes within the PS programs, the display and the PC software has a few more options dedicated to the F90X.