Yeah... but they're both retro throwback platformers that use a "jump into the background" gimmick. I totally respect that there's a difference, but right now I think I'd rather finish up Risky's Revenge than get into another platformer. Unless word comes back that Mutant Mudds is just unmissably perfect or something.
@kriswright You probably should finish Risky's Revenge before buying another expensive download, but Mutant Mudds is more like Wario Land, an action game in which the playfield has multiple planes. Shantae uses the plane-jumping gimmick more for traversal.
Speaking of plane-jumping, I just tried the Guardian Heroes demo on XBLA. What a weird game.