Strong recommendation: find a good supplier who stocks both and will allow you to run some test scans, and try out both units using your own negs. The response of different scanners to retained-silver emulsions is very dependent upon light source, and the Minolta 5400 (which I have) produces some very hard, grainy, contrasty scans thanks to it's hard, cold-cathode light source. The Grain Dissolver feature of the 5400 works to some extent, but the change isn't very great. The LED-based illuminants in the Nikon may prove to be less severe, but note that in general Nikon scanners are still considered to be quite hard on the grain.
If you can afford it, the forthcoming 9000-ED (like the outgoing 8000 model) still uses LEDs but apparently provides considerably more diffuse illumination. Also, don't dismiss the new Epson flatbed, the Perfection 4870. I tried the previous model (3200) and while it didn't meet my requirements for colour and was a bit unpredictable for B&W, the best of the B&W scans were creamy smooth and delicate in their tones. If the new unit actually include proper exposure control rather than purely post-scan adjustments, it might be worth a look.
In saying this, I should add that I'm not trying to suggest that a scanner is necessarily bad because it has a hard light source; it's all a matter of what you're going for. If you plan to shoot Tri-X and want hard, gritty prints in a 1970's photojournalistic style, then the Minolta may suit your purposes just fine. Also, it is possible to calm down the contrast of Minolta scans with a bit of judicious curve control, although there's not much that you can do about its inherent graininess other than use a grain-reduction program (which can have side-effects you may not want) or just bite the bullet and apply a bit of blur to the file. However, life will be a lot easier if you buy a scanner that churns out a look that you find pleasing right off the bat, so it's worth taking the time and trouble to do some evaluations on your own negs.
-= mike =-