N64 for the absolute classics like Zelda OoT and MM, Super Mario 64 but it was also amazing for all the awesome multiplayer games like Smash Bros, Mario Tennis/Kart and Goldeneye. N64 was great for when buddies where over. I still will break it out when I have some friends over for some Smash Bros.
PSX was more of a single player system, for me at least. You had all those awesome JRPGs like Final Fantasy, Chrono Cross, Xenogears, among others.
I cant really pick one over the other; it depends on my mood and what I feel like playing. Good thing I have em both