I've had the N64 and the GC. The PS3 has WAY more 3rd party support, and even if it drops off, it will still be better than N64 support. If the PS3 has the types of games you like, then no reason to sell it.
EDIT: there was a total of 387 games (or really close to that) for the N64. The PS3 has surpassed that number already and will continue to grow a good library.