More Mortal Kombat
The N64 controller is less suitable to play MK then the supernes' controller, but the graphics are a bit better, though it's nothing special for the n64.
The main reason to buy this game is Mortal Kombat 4. MK 4 went 3d and basicly sucked. With MK trilogy you can still play 2d MK on the nintendo 64.
If you were to play Mortal Kombat trilogy these days, you'd still have a good time with it, playing against a friend for half an hour or so. It's very nostalgic.