If its for overclocking wont make much difference its all luck of the draw
I've owned both had 2x Gigabytes then got 2x Asus DCU2 (non top) and in the end i returned the Gigabyte because the 2 i received were poor clockers while the Asus ones are great
But on this forum and others some people have great clocking Gigabytes but poor clocking Asus cards
As for the cards themselves the asus looks better and appears to have better build quality mainly because of the back plate and no plastic cooler
doesn't mean the gigabyte has poor build quality it just doesn't feel as good as the Asus
as for noise and temps the Asus is quiet even up to 70-80% fans but the temps are around 5-8c higher than the Gigabytes
The gigabyte is nosier and has lower temps but its still not that bad still a very quiet card
i had mine in SLI so my temps/noise might be different to yours if your only running a single card
in my opinion if you dont care about overclocking the get the gigabyte as its faster at stock
if you want to overclock then its luck of the draw and wont really matter what you get
flashed my non top with the top bios and then pushed it further to 1080 on both cores with a boost of around 1270-1290 for both cores and 7ghz on the mem
3dmark on the gigabytes
http://3dmark.com/3dm11/3438492 P14615
3dmark on the Asus
http://3dmark.com/3dm11/3479428 P15434
using a i7 2700K @4.8
all things being equal OC wise i think ill probably still pick the Asus due to prefering the look and feel of the card
but you cant ignore the Gigabyte UK returns
ive never had a problem with any Asus part so dont know what the UK support is like