It's close, but taking their average Mhash/s rates from MHC at each sale price, the 5870's 393.46Mh/s divided by $189.99 = 2.07Mh/$

The 5830's 286.45Mh/s divided by $129.99 = 2.20Mh/$

They are virtually identical in terms of performance per dollar, with the 5830 having a slight advantage.