The switch from N64 to PS is talked about a lot - Square definitely started development for FF7 on the N64 prototype systems before switching over. Hiroshi Kawai, a character programmer for Square, has what I think are the most telling comments:

"I kind of had a suspicion that things weren’t going too well for the 64 at that point, because … one of my responsibilities … was to write performance applications that compared how well the 64 fared against the prototype [PlayStation]. And we’d be running parallel comparisons between the [PlayStation] where you’d have a bunch of 2D sprites bouncing off the screen and see how many polygons you could get within a 60th of a second. And even without any kind of texturing or any kind of lighting, it was less than 50% of what you would be able to get out of the [PlayStation]. Of course, the drawback of the [PlayStation] is it didn’t really have a z-buffer, so you’d have these overlapping polygons that you’d have to work around so that you wouldn’t get the shimmering [look]. But on the other hand, there was no way you’d be able to get anything close to what FF7 was doing [on PlayStation] on the 64 at that time."

Kawai has more on the topic, which is fascinating. And the entire piece is just amazing, and I imagine most people on this thread would enjoy reading it.