Whichever one is cheaper. Both will do DivX playback just fine. The Radeons from ATi generally have better image quality. If your playback is fine now, I'd probably wait until you get something faster. On the other hand you can go and upgrade, you might consider a little extra RAM instead.
A video card won't hurt, but that may not be the best upgrade you can get yourself right now.
If you can find a K6 550 (providing your motherboard supports it) for cheap and add more RAM (128MB of PC100/133 is pretty cheap less than $25).