go for the compromise: 5850, best MH/$ ratio. 5830 is ok, but you can't overclock it as much (in terms of %)
I would have agreed with you two weeks ago, but nowadays, it's about impossible to find a 5850 at a reasonable price...
5830 = 300MH/s for $109
5850 = 370MH/s for $187
Yeah... pretty easy call right there, unless you're running out of space.
The one downside is that the 5830 actually consumes more electricity than the 5850, so that should be taken into consideration if you live in CA.