They are both almost equal. Tom's chart's puts them as equal. I think you need to focus on the benchmarks of the games you play most. The 6850 is newer technology that is cooler and more energy efficient. It also scales better if you ever wanted to crossfire. Between the two, I would personally choose the 6850, but I think the 6870 is worth the extra money.
What resolution do you play at? What games or applications are most important to you? How much did each of them cost you?
If you have them both, why don't you just benchmark them yourself on the games you play?