PSX games in general had higher frame rates, more detailed geometry and higher resolution textures compared to N64, although things like perspective correct texturemapping and, in some games, actual tri-linear filtering god dammit, went a long way to make up for the deficit. A game like Twisted Metal was just ridiculous with the way textures twisted all over the place, and since PSX had wonky or non-existing clipping, the way polys tended to simply disappear completely when they hit the screen edge was also quite painful.
I wouldn't really put one above the other in graphics, really. They both had their strengths and weaknesses overall, but on the whole I would probably give the victory to PSX. N64 games were often juddering and blurry, few seemed to reliably hit even 30fps and more than a few had ugly mailslot-like borders in their PAL release. PSX (which I did not own, by the way) gaming was generally more fluid and responsive, giving it an edge IMO despite lack of now considered cruical technical features. After all, gameplay is king, to quote John Romero I believe.
And yeah, PSX software support was obviously far, far superior. N64 didn't even get a Metroid installment for chrissakes...
If Nintendo hadn't gone and fscked up the N64 DD, maybe things would have turned out differently. It'd been interesting to see the alternate universe where that thing saw worldwide release, and watch how it all turned out.