I don't know all the differences, but try looking for how adroit each of the cards are i.e. if an HIS HD 6870 and XFX cost about $200 each, then try looking at the better stronger card for the same price. 6990 is a killler but 6950 is the best up to 200$. In Slovakia there are half price for 6990, from 995 Euros to 575 Euros.
I would get 6990, if I were you so that it will last for a long time, unless you are really pressed on your budget then 6950 would be awesome.