I like 'em both. But I like the Mario series more. In the older days, the gameplay was pretty similar, get from one end to the other. Main difference that with Sonic there was speed. And that was fun. But with Mario, there was the thinking aspect "how should I time my jumps for this?".
That thinking aspect is still there, and I like that now with Mario. I don't like the new Sonic games. Out of the 3D ones, onlt Sonic Adventure 2 was good, and only Sonic and Shadow's level at that. And even then, it was still just getting to the end of the level, but with speed. Basically the 2D games, but with the camera behind you rather than a side scrolling view. You have the occasional freedom of movement, but most of the times it's "go straight and fast".
With the new 3D Mario games, you have full 3D environments of exploration and objectives to complete, coupled with the tradition hopping and bopping Mario fun. I applaud Super Mario Sunshine for throwing in the side scrolling action occasionally. Like the 3D Sonics, these levels were basically side scrollers with a different camera angle. They were much fun and some of the more difficult parts of the games.
I give my vote to Mario. He may move slow, but use that extra time to explore and think, enjoy the game and have fun. ^^;