I purchased the 6950 on a sale for 189 - Never looked back. If you can get it cheaper, its worth the buy. Idles at really good temps, No issues. Plays everything solid without OC. Yes, the 7850 is newer, runs a bit cooler etc... But the 6950 has a bit more raw power to it, 1400 stream processors apposed to 1024. Perhaps it doesnt matter? I think it does.
Both are equally good choices.... I'm looking to see for another 6950 on a flash sale... 150 would get my attention for a second one before its phased out.
Comparing the number of stream processors is irrelevant across different architectures.
For example: My cell phone has 4 cpu cores, as does my computer. Even so, my 2500k completely obliberates the cpu in my cell.
If the price is the same, I can't come up with a single good reason to go with the 6950 instead of a 7850.