I realize that I didn't say which I would get, so let me say I'd get the 6870. I'd get the 5850 if I could find one on discount.
One thing that I'm wondering about, the article I linked said the 6870 used long word 5 and not long word 4. I thought the 6 series switched over and used LW4 instead of LW5? Does only the 69xx cards use LW4?