Megaman of the SegaTech Forums has posted these pictures of Sonic Advance he has found on a website. Problem is, we don’t know which one! But they do look quite nice. The sprites look a little bland compared to other Game Boy Advance games. It also shows that Amy and Tails are avaliable for play.
The pictures were originally found on this page, beware though, the page is full of pictures, so it may take a while to load (includes pictures of Shenme 2, Bomberman DC, Jet Set Radio Future etc)