Unfortunately, the 566 probably offers a better chance of hitting the 100Mhz FSB than the 600, but the 600 would obviously be 50 Mhz faster at that FSB--850 vs. 900. The 566 would be the conservative route, better chance of a successful overclock, and if you get lucky, you may be able to go higher.
Another option would be to hunt around for a guaranteed 600, but these can get kind of pricey. Try a search on Compuwiz1 (username)--he sells 'em, but his experience has been a little dicey with the cellermines. You may wish to consider a PIII if it is compatible, they also perform better at equivalent clcok speeds.