This topic is lol.
Chao_Fan and Siavash, do you *seriously* believe Sonic isn't a kids game while Mario is? Let me give you a little history lesson. Sonic was created to be Sega's flagship character when the key demographic was the under-18 male. These days, the key demographic is the 18-25 year-old with money to spend (still more male than female, but that's changing). Why was he successful among said age group? Precisely because we thought he was cooler, less "kiddy" than Mario. That didn't make it any less targeted at kids than Mario, just marketed differently.
The same is still true. Sonic sells in today's key demo because we grew up with him, and sells in the "kids" demo for the same reasons it always did. If Sonic 06 didn't have Sonic in it, nobody would have looked twice at it, and that's a fact - look at how many new game characters in his ilk spurn a decent title, some mediocre ones and then vanish into obscurity (not that I'm saying Sonic 06 is great, mind).
Don't believe me? Then explain why Mega Collection et al sell so well.
My main reason Sonic was better than Mario, other than the aforementioned mass-marketed coolness? Gameplay. Unfortunately, that advantage was lost in the transition to 3D; Mario 64 and it's successors are some of the best playing games ever, while I'm constantly frustrated by the niggles in 3D Sonic.