DFI if you want to get the best possible overclock for your hardware, and don't mind spending a lot of time tweeking bios options. These boards can be a bit fussy, but once everything is finalized get some really awesome results.
ASUS probably won't have quite the same overclocking potential, but tend to be very stable, while offering some decent overclocking options. If you're looking to do a quick and reliable overclocking job for everyday use, this is the way to go.