Both were the turning point in the Mario series. Mario World added several new features and made the game alot better. Whereas Mario 64 managed to make Mario a success in 3D. Which do you think was more revolutionary and more enjoyable?
Both are badass, and I'm not sure I could pick one over the other. M64 had some really kickass levels (Both of the snow levels were awesome, Tall Tall Mountain kicked ass, Bob-Omb Battlefield was fantastic) but SMW had the Koopalings, and I sorely miss the presence of those little fuckers.
Although, they did make a cameo in a rather recent Mario game...