In terms of amount of content, the N64 probably has more, the PS1 version omitted and modified levels so it could handle the game.
The N64 version definitely looks better, the N64 being the more powerful console when it comes to graphics, and Rayman 2 is compatible with the Expansion Pak, so you get even better textures on the console if you have one.
That being said, the PS1 version is certainly no push-over, due to the changes, it has some unique level design (particularly in the Sanctuary of Stone and Fire) which won't be found in other versions. It also contains some nifty bonuses, cutscenes have voice-acting (whether or not it is actually good is up to the listener), the cutscenes where Rayman needs to have a specific amount of lums to pass are changed to have more interesting dialogue, where the Teensies inform Rayman of current affairs (and even crown him as their king!), which I think is a nice touch. It also includes a few changes in characters, such as in Umber, who readily helps you, rather than attempting to pathetically drown you, and adds some new ones, like Ninjaws.
And of course it contains the Rayman 2 prototype if you get 90% completion or higher.