Generally I think the Sonic games were more fun then the Mario because Sonic games are more faster paced, duh.
I loved Mario RPG though that's still one of my all time favorites, I must have beaten it like 3 or 4 times now.
I remember there being a Sonic RPG that someone was working on a long time ago, but was never completed, so I think a Sonic RPG would be cool.
A long time ago I made a crappy game with Sonic and Mario, where Robotnik and Bowser teamed up, it's was quite amusing, but I lost it when my old computer died. :(