There is never going to be a right or wrong answer to this question because it is simply a matter of people's opinions.
Having said that, if you want to influence people into your way of thinking, it simply isn't good enough to say "Sonic is better, end of story!" or vice versa.
If you said something like, "Sonic is better because the speed of the game adds to the excitement. He has the ability to go SuperSonic, the enemies are better loooking" etc etc etc at least you're trying to justify your point of view.
Otherwise it really just dound like a bunch of 5 year olds in a play ground! Come on chaps let's have some proper reasons!
Personally I like the look and feel of the Sonic games better. The 2D versions of the game are far more fun to play than the later 3D ones because you seem to miss so much of them due to Sonic's excessive speed. There are some interesting levels on the 2D that do prove to be quite a challenge, and the boss battles are fun as you work out it's path and them destroy him.
Mario on the other hand is a small fat Plumber, but there seems to be more to find and discover in his games. Giving it a larger feel. The ememies also appear to be smarter and more of a challenge to defeat. Also as he moved into the 3D versions, there's definitely a lot more to do and hense more fun because it's at a pace that you can handle. And boy, Yes! Is his misses ever way out of his league... And besides he keeps on letting her get kidnapped all the time... And then there's the dreaded film version... But let's not speak of that ever again!!!
Two exceptionally likable and interesting characters, Mario gets the edge for me!
PS: This is in the wrong forum anyway! Check before you post yeah