SEGA of Japan has put up their website for Sonic Generations. They have screenshots of the HD versions of the game, very nice and fancy stuff.
Here is something odd. The HD versions of the game say they are single player titles, but the 3DS lists 1 to 2 players. The 3DS already confirmed it will get an exclusive level, will it also get exclusive multiplayer?
[Source: AndriaSang]
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I think they are going for the Sonic 2 two player race mode. Just a guess, but it would make sense.
Also, I think 3DS will have a completely different list of levels aside form Green Hill, which both versions will share.
The Nintendo Power article already confirmed that the 3DS version will have multiplayer.