I would go for the Abit NF7-S but it is case by case, the Asus COULD beat out an NF7-S just like the NF7-S COULD beat out an Asus. But the favorit on this and most other specificly OCIng forums I have seen is the Abit.
i really like the ASUS, but consensus is to get teh Abit...its one of the best boards, though it does have a high return rate (but again, its one of the best boards, and the 2500Barton is an excellent cpu!)
i have had both.... the only real difference is the abit will let you adjust your chipset voltage, the asus will not.... but the asus does have a few more features.... you can see in my sig what i stuck with..... i love it! only other board i would concider would be a Epox 8RDA3+ but you may end up with a rev 1.X with that