5850s are not as efficient as 5870s. Less stream processors is my guess as to why. You can get four 5870s on one PSU, no problem.
5850s are significantly cheaper to buy though.
You would save money on the MB if you went with less 5870's.
For me the 5870 is the sweet spot. I would love to have a 5970 but I never run into them cheap.
Where can you get 5870's for a reasonable price though? Everywhere I'm finding them they're 300+