actually, the Asus board will likely get better overclocks. Its FSB speed is adjustable from 200MHz to 400MHz in 1MHz increments, allowing for a 100% theoretical overclock (of course, no chip can currently go that high, and i doubt the board would be stable, but still). Also, DIMM voltage goes higher than 2.8V, which is the max for gigabyte on these new boards. And it includes nvidia's ntune software, which is nice.
Still, i don't want to knock gigabyte. From the benchmarks i've seen, it's a great performer, substantially outperforming the reference board (at least in the benchmarks of the K8NXP-9 i've seen, not sure about the SLI), and it's stable. Feature sets are pretty much identical, right down to the dual GbE, Realtek audio controller, etc. One thing to look out for, though - apparently, gigabyte has delayed release of its SLI boards till early january so that they can do some bios tweaks. As such, if you need a mobo soon, it will likely be an Asus. Otherwise, if it's january, then i'd just go with whichever board suits your tastes.
The only board i'd avoid is the MSI one. The lack of PCIe x1 slots just pisses me right off.