SNES sound vs Mega Drive sound Watch
Earthworm Jim 2 (Lorenzen's Soil)
Mortal Kombat 2 (The Dead Pool)
Mickey Mania (Final Boss)
SNES: http://www.youtube.com/watch?v=rcRTByliq_k&feature=rela ted
Mega Drive sounds better for the games you mentioned. Street Fighter 2's soundtrack sounds like crap on MD, though.
The Megadrive's processor was twice as fast as the SNES's. This meant that even though the SNES might be able to have more sprites on screen to do so it might have to have some bad slow down (as I remember with Smash TV). I never experienced slow down on the Megadrive.
To use a modern day comparison, the SNES was like the PS3 and the Megadrive was like the Xbox360. The SNES was capable of the very best graphics (like Donkey Kong Country) but, in practice, the clever artists at Sega and their partners, like Shiny Entertainment, ensured that the Megadrive competed well and often better, such as the Megadrive version of Aladdin (better looking and smoother than Capcom's).
The more limited capability of the Megadrive sound chip arguably suited the console which was partially going for a more direct, funky feel with Sonic the hedgehog , Streets of rage, Toe jam and earl etc. Some tunes did sound too crudely snythesised and tinny though.
The SNES's chip aided its typically Japanese orchestral musings. I was much more of a Sega fan.