G31 is a very good board for the price. I have the Gigabyte with a E5200 & HD4850 and performs on par with boards over a R1000 more. If you don't need the extra features of the more expensive boards don't waste your money. Only weekness I can think of is not a great overclocker(to be expected in this price range).
I am more familiar with ASUS in general, so I would lean towards it. Memory graphics card compatibility should also be marginally better with the ASUS, since ASUS ships much higher volumes than foxconn.
Also, I can confirm that the ASUS's lan adapter does not have the slow Vista network copy bug. My friend can copy from my PC at a nice 78MB/s, where his hard drive cant go any faster.