@nut : well Nations only contains one environment, Stadium, and TMUF has stadium + 6 environments in previous Trackmania games. And each environments has a specific kind of car (which greatly change the handling) and specific sets of blocks. So yeah, you could call it an extension (I think you can even use a TMNF account in TMUF). Even though Stadium is the most polished environment, I think TMUF is worth its 30€^^
And there's plenty of downloadable content for it, so changing car skins/models isn't really a problem.

Damn, you've given me the urge to get back to it... I have to resist, that game was really addictive^^