At $150-160, those are both good deals. The 5850 is slightly more powerful than the 6850, which garners Tom's Best Graphics for the Money this month at the $175 price point. Depending on the review and the particular benchmark, the size of the performance difference varies (the 6870, 6850's big brother, is the intended successor for the 5850). The 5850 uses more power, but should also be more overclock-able.
I use 2 5850's in crossfire and am very happy with them.