You didn't specify whether you meant the ASUS Strix Gaming or the ASUS Strix DC3 OC, so I am going to assume you meant the Gaming model, which doesn't have the more aggressive factory overclock.
Overclockers have published the results of their look at this question:
In short, as I read the results, the ASUS Strix GTX 980 TI's and the Gigabyte GTX 980 TI G1 Gaming's fps are almost indistinguishable in the games tested at clock and memory base rates. But when the Gigabyte is run at overclocked rates, it pulls 5% higher fps compared to the ASUS Strix.
(I'm pretty sure that that small gain would vanish if the comparison were the factory-overclocked ASUS Strix vs the factory-overclocked Gigabyte.)