At stock 570 is 5-10% faster. However, once oced, 7850 is up to 10-15% faster as of now, more like 10% oced vs oced but with time that difference will increase VRAM demanding situations, especially for min FPS/smoothness.
If you can get a GTX 580 under $280, then that would be a comparable deal. Though you would still need to be able to afford a better PSU, more power, more cooling since the heat is much more, and still VRAM limitations. So ideally assuming both are $250 they are about equal, 7850 and 580 since both are comparable once oced but 7850 has more VRAM and is more future proof etc. However, 580 up to $280 can be considered.
$300+ for 580 OR Rs.250ish for 570/6970 = NO DEAL