Same chip, just different manufacturers.
You need to just pay attention to the specs. One may use better g-RAM than the other. Also, one will have different alternate add-ons than the other, which may change your decision.
I will note, though, that Giga-byte often sells good graphics-card packages with great stuff, but the heatsinks they use are often cheap and shytty. Not to mention that the boxes from Giga-byte usually smell like the rotting putrescence from the severed hands of the 4th-world country workers they use to build the stuff.
But if that doesn't bother you, then Giga-byte is a decent choice.