I will usually go with XFX, the double warranty means it is transferable if you ever sell the cards to someone else (or give them to your brother when you upgrade). Which if you have a short upgrade cycle is key. However, sapphire tends to release nicer non reference models.
However, if we are talking about the stock, reference, all look the same cards then they are all exactly the same in terms of what you get in the end. Get the cheapest one you can find.
If you want the reference card, get xfx for the warranty and excellent RMA support. If you are looking for fancy cooling with crazy OCs XFX rarely releases these kind of ATI cards and sapphire's are usually the top of the line. XFX will release higher clocked cards (black edition) but almost never with better cooling. Mind you XFX also covers changing the heat sink and over clocking in the warranty..