Honestly, I think it depends what you're rendering. On any scene including anything metallic (and most scenes which use Material room textures without any imagemaps attached to the nodes) the firefly renderer looks better. Most of my custom props are textured entirely using P5 Material nodes, not image maps, and I almost always use Skin nodes on my figures now (which I don't believe the P4 renderer can handle), so I see much more benefit in using the firefly renderer exclusively.