I have to say Mario because I've enjoyed Mario games more. Sonic games were just too fast and most of the time just seemed to be the same thing over and over. The only level I really liked in Sonic 2 was the oil factory one because most of the level wasn't running around but more of a plat former.
Also, Mario has stayed true to his roots over the years, every Mario game feels like a Mario game. Sonic games have gone through some weird stuff, their RPG didn't seem like Sonic as much as Super Mario RPG, Partners in time, and Bowser's inside story seemed like a Mario game.
Mario games however are simple compared to Sonic games. I'll use the RPG element again, Sonic Chronicles had a much better story line than any of the Mario RPGs.
I like Sonic games but I just like Mario games more.