According to both games Sonic and company are on Earth. There is a lot of debate on the Earth/Mobius topic.
Does Sonic Adventure 2 battle take place on Earth or Mobius? I've heard them menchin Earth a lot and also there are ALL humans living there... I've never played Sonic Adventure 1, do they travel to Earth or something, or is it one big mistake made by sega... (God I'm such a Sonic nerd *sigh*)