Until now they've all been made exactly to the reference specs, though there will soon (no one knows when, but it has been officially confirmed) be a "vapor-x" version (better cooling, thus more overclocking potential) by Sapphire.
Right now performance will be exactly the same between the brands, but I've heard XFX has better warranty in the US.
The same thing happened when the ATI Radeon HD 4770 was introduced earlier this year. The cards were all built to the ATI reference specifications. It was a few months before XFX came out with a 4770 with a different fan shroud that exhausted warm air out the back of the pc case.