They are all built from identical specifications, so the difference is made up of value-adds. Some manufacturers will pre-overclock their cards, add custom cooling and bundle games or other software with it, obviously at a price. So be on the lookout for this when choosing a card. A 5% overclock does not justify a 20% price bump over the stock model.
There are really the same cards, like it was said, there may be a small bump in speed for a waste of money. if anything check out the manufacturers warranties. A lot of people go to different brands depending how their warranty is.