Sonic won't be quite the "fastest thing alive" for Sonic Generations on 3DS. While the PC, PS3, and Xbox 360 versions are all launching on November 1st, the 3DS version will be three weeks tardy. Sega has announced that the 3DS version will be available on November 22nd.
3DS fans shouldn't fret too much, though. Sega has reconfirmed that the levels made for 3DS are "designed exclusively for the Nintendo system." With the exception of Green Hill Zone, the two games draw from different sources: the console games from classic console titles, while the 3DS game adapts previous handheld stages. And yes, you'll be playing as both Classic and Modern Sonic in the 3DS game as well.