I don't mind people writing this and that about the rivalry between Sonic and Mario, but most people (most!) seem to assume that it's the only way to go. I mean, just because the two (and soon, three) games they've appeared together in are centered around competition doesn't mean it's all they can do, does it? I for one am a firm believer that they can bury the hatchet and fight alongside each other, rather than against each other.
The Mario's sprite from Mario & Luigi: Superstar Saga; I downloaded it just for this stamp. The Sonic sprite's from Sonic Advance 2; I had it saved on my hard drive for who knows how long.
Just because we've never seen it happen doesn't mean it can't. And I'm absolutely sure it can. Who's with me?
Sonic, etc. © SEGA
Mario, etc. © Nintendo
Note: I did not know of Super Mario Bros. Z at the time that I made this stamp. Now, thanks to those who have commented on it, I am very adequately informed.