5870/5970 are more efficient than the 69xx/6990.

The cypress core has 1600 SPs vs 1536 SPs, which is what gives them the edge.  Yes the SPs are different between the 2 cores.  The Cayman core is ~25% bigger than Cypress, thus its power consumption is higher.  Sadly the supply of 5870s and especially 5970s is pretty much gone.  You can pick up 5870s on ebay for cheaper than the unlockable 2g 6950s, so I'd say the used 5870s are a better plan, especially if you get reference design cards.