Honestly I grew up with Mario's NES games and it didn't take me much to find Sonic's Genesis ones, and even tho I can see why people like them I've never grown too fond of them, and thought that even tho they're good they're probably not the best the Genesis can offer ... tho the same can be said of the Mario games... hmph... well, except maybe for Mario 3, of which its length and content are impressive for an NES title.
As for their 3D games I only got to play Super Mario Galaxy and I loved every single minute of it, and at the same time got Sonic Colors which-... I actually got to despise, really. Its trailer was great, but I found its length and simple gameplay to be kind of insulting in comparison ... but hey, I actually loved playing Sonic Unleashed, so maybe there's something wrong with me, who knows?

In conclusion: Wahoo! > Fast