Rayman is very creative in terms of presentation and has better looking graphics, is better in multiplayer.
On the other hand Mario doesn't feels sterile, and multiplayer is a bit of a mess, still the game mechanics are more refined and level design is much better.
If you're looking for something fresh and you want to feel the sense of discovery and variety I would go with Rayman, if you're looking for good arcade-feel platforming I'll go with Mario.
Still Donkey Kong Country blows both of em' out in the water.