5870 is best hash per ££££($$$$) or the 5850 OC'd. 5870's are like rocking horse poo to find, and 5850's are more and more the same. 5970's are still silly money, especially with the dip in BTC, but hey, thats half the fun 
How do you figure that? How many hashes per 5870 and what price are you getting them at?
Example:
$280 5870, 460MH/s = 1.64 MH/$
$400 5970, 700MH/s = 1.75 MH/$
Unless you are finding 5870 at less than $250 per card new I don't see how they are best hasher per $$$.
Are far as the 5970s being "silly money" unless you are only buying one card, 2x 5870 cost more than a 5970.
5970 makes deployment simpler and has higher electrical efficiency. However keep buying those 5870s I don't want the price to go up on 5970s.