although i cannot speak for the 980 specifically, i do have the 970 G1, so i will use my reasoning based on the 970, but teh 980 probably wouldnt be much different other than the specs of course.
first, i want to mention that it probably will come down to personal choice when you make your pick as the differences between them have their ups and downs.
the gigabyte cards (again, going off of the 970) take more power, which means a couple things. higher OC threshold, but also more heat. some will say that quality is a concern for them, and that the heat is part of the quality concern. in the end, will this matter? if you dont OC, probably not.
I personally chose the G1 because, by stock ratings, it was the better performance card, although it wasnt much. it was also higher cost for the slightly higher performance increase.
cards such as the msi have better cooling. i am a big fan of MSI, so i believe their quality to be good. their card speeds arent as good as the G1, but again, it isnt that big of a difference to make much of a difference unless it all comes down to epeen size. This also really comes to be the same with the EVGA where there wont be a HUGE difference between them.
under heavy load, my card gets to temps in the 50s. i cant see the 980 being much higher. myself personally, i dont believe that there will be quality problems, as i havent seen anything for this specific card floating out there.
if you decide on the G1, i think you will be happy, but on the same token, you will be happy with the others as well. i love the G1, but im sure i would be just as pleased with an MSI or EVGA too.
edit- i also looked at the physical construction too. if i recall, the msi didnt have a backplate to help with rigidness, whereas the G1 does. will this matter in the end, maybe, maybe not.