As a gamer, I'd say both systems are needed in one's collection.
Technically, the SNES was a bit superior to MegaDrive/Genesis, however some games that exist on both systems were better on one or the other, depending of the game.
Some exclusive MegaDrive/Genesis games were remarkably well done. VectorMan, Sonic, Comix Zone, Contra...Final Fight CD, All those Sega CD RPGs...
Same goes for the SNES. DKC series, Marios, TMNT, etc.
Like I said, a real gamer needs both systems. :-)