The Mega Drive version has a number of differences: "realistic" sprite work (compared to the "cute" style of the original), brand new music, a reworking of the secondary weapon system, the removal of the autofire powerup, and new stage sections, among other things.
The Mega Drive version of the game includes various improvements over the original, such as more realistic visuals, new musical tracks, an altered secondary weapon system, the absence of autofire power-up, and new stage sections.