I have a built-in bias in favor of Nintendo just due to circumstance of having an older brother who had and favored Nintendo. So the SNES influenced my gaming preferences from a young age, and I'll always have more nostalgia towards it. Although we actually ended up getting a Genesis, we kind of neglected it. I didn't really know a whole lot about it until my collecting years. I think it has a damn good library of games and I can see how someone might favor it if they were playing Toejam, Golden Axe, Streets or Rage, Shinobi, Rocket Knight, OutRun, and a slew of legendary shooters like Musha, Truxton, Thunder Force, etc, not to mention lots of other great ports and exclusive counterparts to SNES titles. I wish I had played all that stuff back in the day.
Holy crap, I just discovered that I own two complete copies of Altered Beast.... I never realized that. Anyway....
One admission I will shamelessly make in favor of Genesis, is that I like Sonic 1 thru 3 hella better than Super Mario World. However, because my childhood memories are closely tied to playing SNES, I'll always favor it.