Sapphire is the largest manufacturer of AMD cards and has the better reputation. I have used several in the past and they were good cards. Never owned Powercolor and probably won't in the future either.
Its pretty simple. If you go AMD, Sapphire. If you go NVidia, EVGA. I have had Sapphire since my 9700 Pro and not had one card fail me yet (9700 Pro, 9800XT, X850XTPE, HD2900 Pro, HD4870, HD5870) and now have a HD7970 Dual X and Vapor X and HD7870 Dual X.