Probably 7850 on most counts, but if you go LTC instead of BTC, the 6970 should be close to the same performance (500KH/s).
Well good sir, you have to explane to me how 500kh/s would be possible on a 7850.
I got 2 of them running @1125 core, 1500 mem with 19intensity and they both get 390 kh/s.
And 1125, 1500 is a pretty high OC for that card.
And even 390 is pretty high...