I just bought the full version right now. My wallet cringes for I am a terrible wallet father.

In other news I am doing a happy dance.

rami my account name is crumply if you wanna race

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.

Originally, United had more Stadium blocks, but Nations Forever gave Nations the same blocks and all

As for the United Forever graphics, it's the same as Nations Forever.
Which means pretty good. ;d

If anyone wants I can upload some awesome custom tracks compatible with Nations and United (here or in a premium place...). You will tear your hair out trying to complete them but I've played them a lot on multiplayer with the guys and they're hella-fun. I also made them.