The XBLA game has tons of improvements, including proper widescreen support, visual filter, US and Japanese soundtracks, the ability to unlock Tails as a playable character, the option of using the Sonic 2 spindash, and Achievements. Also the music loops properly and load times are quicker. Come on now, you really have to ask?
QFT. This is a great value. I bought it even though i have sonic gems collection.