I say Asus.
It already has a bios that allows a higher oc than Sapphire or XFX.
Sapphire or XFX bios only lets the core go to 950.
Asus (I cannot remember how far exactly) but, it is higher. Above 1100 I do believe.
You can put the Asus bios on any other 5770's but, flashing is always dangerous...
Oh, and try to get the V1 cooler.
Not the stupid Egg thingy.