First let me qualify, this will be for varmint hunting and probably never actually get used in competition!
For pure accuracy potential, which of these two rounds would you prefer?
Thanks in advance - memtb

The 22-250 has proven itself to be highly accurate for many years. However, I don't see any reason why the 223 WSM would also not be very accurate. If you want the ultimate in accuracy, you should probably build a 22 PPC or even a 6 PPC.

depends on what you want I have both and love them equally. The 22-250 is a little user freindlier in the fact that it doesn't copper foul as bad. Both shoot 1/4 inch at 100

.22-250 or a .22 BR. The .223 WSSM seems to have some negative press over on benchrest.com in the forums; probably tied more to some of the rifles it's offered in and the brass available, but I think the .22-250 would be less headache right now.