I have beaten both of them. The PSX version has a few cosmetic improvements over the Saturn version. Some transparencies look better (but do not be fooled: the Saturn version is full of true transparency effects), with a brighter look to them and little extra effects on some spells.
There are a few (and I say again: a *few*) gouraud shading effects in some areas, but they don't enhance the game too much (they could've used it everywhere and really boost the graphics, but it was probably too much work), and does introduce bad dithering where it's used (the rest of the game shows no sign of dithering at all - a good thing).
Some things are not as good, however. The sprites look a bit pixelated (it's as if they don't match the screens pixels at the default zoom level anymore) and had their palettes changed slightly. Also they changed the characters' circle shadows by the angled shadows that you see during battle. This was a bad move, IMO, since those shadows are merged with the sprites themselves (so they'll stick to the characters during jumps, instead of staying on the ground), and they look awkward when the camera goes rotating.
All battle backgrounds got a lower resolution too. They're blurry and look out of focus, what is a drawback, since the resolution in the Saturn version is good enough so the battle backgrounds look like the real time backgrounds. Also, all "mode7" effects were replaced by simple polygons, so usually the ground looks more pixelated and with a bit of the ol' PSX texture smearing. The water effects looks worse due to the same reason.
Regarding framerate, the PSX version seemed to have as much slowdown as the Saturn version. A few areas run faster on the PSX, but some other ("mode7" heavy) areas actually run slower.
As for converting the audio... all audio (voice and music) in the Saturn version is compressed with ADX. I'm not sure about the PSX version (did any PSOne games use ADX at all?). All music tracks are fused in a single .STZ file (FIELD\GM2.STZ). I don't know if the voices are in it or not.
--EDIT00
I was trying to attach a small app that can play .STZ files, among with most of the ADX-based files you can find in Saturn games (it can also decode the audio tracks for the Lunar2 and Burning Rangers FMV). But I am not allowed to post .LZH files. If anyone wants it, PM me.