This sure is a loaded question....
They are almost all identical you see every brand is using the exact same GPU, they just throw there own twist on it.
More memory is good for larger resolutions, more fans are typically better at keeping it cooler.
Some cards will have different clock speeds, but it's very easy to overclock a GPU with the right tools. (beware that some brands will void warranty for overclocking)
At the end of the day you should really look at the warranty offered and the price.