DFI if you're into tweaking, overclocking.
Asus for features, great performance and reliability. especially the newer models.
personally, i'd recommend the epox 9npa. its a great overclocker and has great stability.
also it is much cheaper than either of those two boards ... its less than $100 at some places.
if you want the absolute best of the best, overclocking, features, layout, style and everything else, then wait for the Sapphire PURE to come out.
it is based on the ATI chip and is superior to other 939 boards (at least based on the numerous reviews of it)