Mario is the game character who single handedly revolutionised the home gaming industry. We would not have what we have today if it wasn't for. Mario. Sonic was a copy by Sega once Mario proved so popular with nintendo.
Sonic moves so fast it does not allow for the exploration you get in mario and mario has had many, many more games than sonic.
I'm not saying like sonic is a bad series of games. But Mario seems to have more things to do in the games than go though loops. Unless It's the Olympic games I'm playing Mario games. Plus mario seems to make you want to play the games more they have so more things hiden than Sonic.
Without Sonic, expect Nintendo making crappy video games during the 90s and probably later on. Sonic was his major competitor and got Nintendo and SEGA competing (Competition is a win for us).
Also for me at least, Sonic has far better potential than Mario. From every Super Mario Bros. Game, all Mario does in every one is capture castles and destroy Bowser to save Princess Peach. Sonic on the other hand adds more characters and plot devices in each game.