The screenshot above taken with 3Dfx The one below with PowerVr. I know that this from different games, but look at the bottom at the pictures: The 3Dfx (above) is smoother even though its only running 640*480*16bit and the PowerVR (below) is running 800*600*32bit. The reason the 3Dfx looks better even in 640*480 is because of the Bi-Linear and Tri-linear Filtering that the PowerVR dosen't have.
This convinced me that the 3Dfx is the best and fastest.