if they both are truly Redbook audio (technically they would be
Mixed Mode CD) then how compressed the music is in the final release has everything to do with the composer/company that made the game, and not the system it was on. I might have to look into it though, since you compared the same game and the Sega CD one prevailed.
did you rip them both yourself? the gamerips online can differ in quality (but I'm sure you know that :P)