yah, SSF uses pure software rendering. It's slow and will look pretty pixelated, however, it will result in an image that is perfectly identical to what a real Saturn would display (your TV meshes the pixels together anyway, so its not as much of an issue there).
Girigiri uses your 3d card to do the rendering, because of this it has tons of video bugs and its generally inaccurate everywhere. However the polygons are displayed at a higher resolution, so the polygon edges are less pixelated. imo that makes it look even worse, because the textures themselves are still low-res, so it looks pretty bad in overall. The higher resolution also introduces vomitous pixel scaling as girigiri uses no filtering whatsoever, so unless you set (numerous) costum resolutions on your videocard/monitor matching what your game is using, and tell Girigiri to use that resolution (which needs another hack to activate a hidden resolution changer), 2d sprites and 2d everything will look disorted.