This is exactly what I was talking about earlier. When emulating certain realistic instruments (guitar being the absolute biggest offender), the Genesis suffers big-time. You want guitar samples, you gotta go with the SNES.
I heard some decent SNES guitar samples, but I've also heard some pretty fake-ass sounding samples as well (being the majority).

A few games pulled off guitar sound pretty well on the Genesis. Vapor Trail is one of them (the sound track is even better on the Genesis than on the arcade) - http://www.youtube.com/watch?v=YSssZMGDozU and http://www.youtube.com/watch?v=4IDvmCibcuA

I have yet to be presented with a Mega Drive game with music/sound that has impressed me (the only exception to that would be the Streets of Rage series).
Well, depends on what specifically you're looking for, to impress you. If you looking for a Genesis game to sound like a SNES game, then you totally have the wrong idea. I personally think there are tons of great/awesome tracks for the Genesis out there (old stuff and recent tracked stuff). Most SNES stuff sounds cheesy nowadays, with a few exceptions (notably orchestral stuff). I think the only large advantage the SNES really has nowadays, is that even bad compositions don't hurt your ears the way bad FM can. Most bad snes compositions are just forgettable. Not so on the Genesis, and I think it gets a bum rap for that.