All depends on what you're looking for, here's some of the difference that can be found between venders:
Type of Cooler used
Sometimes alterations to the PCB
Included Accessories / Adapters
Factory overclock (Core/memory frequency)
So you have to decide what is important to you and then try and narrow down the possible list, then when you have just a few choices I'd recommend looking at some reviews of the card(s) to help make your decision.