Usually the difference is so small that it's not a major factor in making your decision. They usually all have different cooling systems and amount of video RAM, and some will apply a small overclock for you, however you can also do this yourself (but it's technically not covered by their warranty).
Internally, they all use the same graphics chip made by AMD, they won't have drastically different performance. They simply slap the chip on a board, supply clock signals to it, and put whatever memory, cooling, and connectors they want to use on it.
You can look at the clock rates of the same card, the ones with higher clocks will be slightly faster (50MHz on a 900Mhz card is about 6% better).
Usually though, things like the warranty, brand preference, and connections become more important than the minor differences in performance.