sonic the hedgehog had more to do with the success of the genesis than smw did with the success of the snes. in 1991 mario was stale, and nintendo was tired of him. in fact, it took nearly another 20 years before nintendo would release another console 2d mario game. the staleness of smw is one of the reasons why the snes stumbled out of the gate after launch.
sonic the hedgehog was a breath of fresh air in 1991. it legitimized sega as a real player in the industry. marketing alone can never sell a game, it can only provide a spark. when a game goes into the sales stratosphere like sonic the hedgehog (pulling up with it the install base of the sega genesis), the quality of the game is the workhorse.
in 1991, sonic the hedgehog was the better game. it sold better than smw, and it pushed more consoles sales than smw. people bought sega genesis consoles to play sonic the hedgehog the same way people bought NESs to play super mario bros 5 years earlier. people didnt start buying SNESs in big numbers until Street Fighter II was released.
in 2011, sonic and sega are laughing stocks and it is easy to forget that they were ever on top of the world. add to that modern gaming sensibilities (games should be easy, games should save progress, ability to grind, etc), then smw is the better game. but an old schooler like myself still appreciates how fun sonic the hedgehog is... with my limited time to play games, going back and whipping through the original sonic and collecting all the emeralds is still quite the challenge.
super mario world feels like a chore, i love to play through the first world, but things start to get dull in world 2. and with the ability to save, and go back and grind for lives and what not, there is no sense of danger of me losing all of my lives.