So yeah, it all comes down to what gameplay style people perfer. If you want more challenging, faster paced platformer with more variety to it's levels then Bros 3 is the better game. If you want a more relaxing experience that allows you to explore more and play at a slower pace, then World is the better game.
That would make sense for my tastes. For me, Mario is great and all but takes a back seat to Zelda and Metroid, which feature more exploration and a slower pace. Hell, when Yoshi's Island came out I was initially bummed out that the level progression was so linear. I loved how in SMW there were multiple paths and secret exits and the worlds were less formally structured (in Yoshi's Island each world as the exact same amount of levels and it's the same amount of levels between the mid-way and end castle). If you don't give a **** about that then I understand why SMW doesn't blow you away like it does for me.
For me stuff like secret areas, exploration or non-linear progression are signs of depth in a game. That's just kind of how my mind works. Those things seem more ambitious than level-by-level straight action games and I tend to instinctively associate ambition with quality. I've learned to appreciate pure action games (especially since they've become rare) but they still seem lesser to me in some way.
I think it's because my brother only plays "dumb" action games. I don't mean "dumb" in that they're bad games and that you're stupid for liking them. I mean "dumb" in that they require little to no thought and you don't have to really figure much out to succeed in them. You succeed a lot on reflexes and skill. My brother doesn't like Zelda or Metroid because he honestly isn't intellectual enough to be any good at them. I think I tend to be somewhat elitist in regards to games that require the player to be smart or clever in order to succeed. Super Mario World will reward a player that thinks.