Sega has confirmed it will release a second demo for Sonic Generations on Xbox Live tomorrow, October 18, and PSN on October 25.
If you played the previous demo you will remember only being able to try out a classic Sonic stage. The second demo will allow you to play both classic and modern versions of the Green Hill Zone level.
While you wait for its release you can see what Green Hill Zone is looking like in a video over here.
It's out in UK on Xbox, PS3 and PC on November 4. US get it November 1, while a 3DS version will be out later that month.