First of all, the ASUS equivalent to this two boards would be the ASUS Sabertooth r2.0, not the M5A99FX that only has 6+2 power phase, and consequently, less OC potential.
Second, but not less important, ASUS is very simlar to Gigabyte respect to materials quality and OC features, so once again, everything relies on the price you can get them by.
My advise? Catch the cheapest one (or the one that fits your needs better), you're not going to see any performance difference 😉