AFAIK, there is very little difference. Basically Ultimate Carnage is Flatout 2 with slightly better graphics and details, but little or no other additional content compared to F2. I think they released UC in order to port the game to Xbox360 and give it a more polished next-gen look. However, I haven't played UC yet, so...
Technically, UC is better apart from the missing LAN mode and the crappy GfWL. But one of the things I love most about FO2 is the awesome soundtrack, and the music on UC is really boring and generic, not the awesome tracks by Wolfmother, Megadeth and Audioslave that made FO2 the game it is to me.
flatout 2 wins by me. Reason? Check here (btw, I wouldn't recommened you carpack 3, because it removes some pretty awesome cars, and not all of the new cars are good). Also, even though server is down, there is lan mode, so you can play through tunngle, hamachi etc. If you can't find some friends to play with ya, I'm always available.
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