So, I got this issue in the mail today (subscribed last year during Amazon's 5$ sale) and here are the things everybody wants to know:
*Classic Sonic is the same as he is in the console versions, that is similar to the Genesis versions with a platforming emphasis. NP says it's more faithfully "old school" then Sonic 4 was, especially in regards to the physics. The 3D is used in this mode, but only modestly.
*Modern Sonic is different from the console versions and is more similar to Sonic Rush/Sonic Colors DS with an emphasis on speed. The 3D is used a lot more in this mode, such as with the first boss (Giant Fist) going towards you. Sonic will also be able to move between the background foreground.
*The Bosses are all unique to the 3DS version.
*There will be some unique stages on the 3DS version - Casino Night Zone and more to be revealed later.
*There will be "Special Challenges" that can be unlocked via StreetPass and/or Play Coins
*There will be a local wireless racing game