To me, Sonic beats Mario. Not to say that I don't like Mario. Super Mario Bros. is a true classic, Super Mario 64 is a great example of an N64 game (and the port to the DS was surprisingly successful), and both Paper Mario Games are two of the best RPG's I've ever played. However, Sonic's just appealed to me so much more since I started playing video-games. Some people say Sonic's been getting worse and worse, and I do agree Sonic Heroes weighs the series down A LOT, but SA2B and Shadow the Hedgehog *GASP!* are two of my favorite games of all time.
Maybe it's just because I enjoy high-speed platforming more than...uhh...regular platforming, but a super fast, spiky blue hedgehog has always appealed to me more than a plumber in overalls and a red shirt, even though they're both cool. Not only that, but the Sonic games just seem more serious to me. Mario is more of a comical hero, but Sonic games have a solid plotline, not to mention the finale's are always awsome, especially in Sonic Adventure 2.
On the other hand, the Mario RPG's DO have a serious plot, they just incorporate humor as well, which only makes them better in their case. But in most of the other Mario games, it's very light-hearted and goofy.
Basically, they've both got their strong-points, and both are amazing series which I hope will continue for many years to come. However, Sonic gets my vote because I simply prefer speed above most other styles of gameplay, and because the first time I picked up a copy of Sonic the Hedgehog along with a Sega Genesis at a garage sale, I knew I was getting into something amazing.