quality...some are better than others and some are just overpriced fluff.
at one time pny was a good brand, then something happened and now they are crap. his was at one time a bargain basement brand but, if you shop carefully they have some decent hardware. brands like powercolor, zotac, visiontek, galaxy, ecs, sparkle...all low end junk. the brand you can generally count on to have good performance, competitive price and mostly reliable hardware are, evga, xfx, sapphire, msi, his, gigabyte and asus. the differences are going to be individual component quality and performance. evga and xfx tend to have the best warranties.