Raw theorical polygon calculation rates for the PSX were 300,000, whereas the Saturn were 200,000, as mentioned previously. So, in theory, the PSX could calculate 50% more polygons than the Saturn in the same time frame.
But, of course, it doesn't only comes down to polygon processing. There are lotsa other factors that affect performance and visual quality. The PSX had the big advantage that gouraud shading was less stressing for it's CPU than on Saturn. It also looked better, since the Saturn uses the weirdest gouraud shading rendering I ever seen. The dithering also improved the quality a bit, smoothing color gradients, and it supported 24-bit textures.
The Saturn, on the other side, could have the VDP2 render it's layers using rotation/scaling, per scanline (good old MODE 7 effect), at little CPU cost (looking a bit prettier than polygons, and allowing deformations, like fake reflective water) and, even lacking true pixel-level transparency, like the PSX, the Saturn had some transparency modes that wouldn't cause any slowdown (overly used in SonicR).
The main problem with most cross plataform games was that they were first made on the PSX, then quickly ported over the Saturn, and were not optimized to use the Saturn's exclusive features. The 24-bit textures were not properly converted to benefit the Saturn's color palette and different texture rendering (this is a key problem), people didn't care to find ways to make transparencies look good, using mesh transparencies instead, and the whole rendering code didn't receive much effort for increased framerate.
But if you look at Grandia, you see that the PSX tasted of it's own poision. It took ages to port it over the PSX, and the port did suffer graphical losses. The textures don't look as good in the PSX (they seems less "stable", and aren't as sharp as the Saturn ones), all mode7 planes were replaced with polygons, making them more pixelated, some areas suffer of lack of perspective correction (but they amazingly minimezed such problem in most of the game), sime minor details were gone (like the characters' sprites getting darker when they are under shadows, or in dark areas, the gorgeous water effects), and the battle 2D backgrounds got their resolution halved (they added a fake filtering that blurs it a lot). Slow downs ins ome areas decreased, yes, but slowdown was still present. The things that got better were the magic spells, that got flashier on the PSX version (but, surprisingly, the Saturn version spells uses a darn good transparency already).