United got (I think) 6 or 7 seven different enviroments, while Nations only got one (the stadium). Each environment has different kinds of cars with different kinds of physics, and completely different kind of driving style to master. United also got an awesome content sharing system, and you get more building blocks for the level editor (even for stadium environment). United Forever (the latest installment) also got improved graphics and engine, but I haven't tried it yet.

big thanx to dragonslayer for sig!