I would have to disagree as for the 7970 being even slightly better. Here's a link to the spreadsheet I made for this, it factors quite a bit in.
https://docs.google.com/spreadsheet/ccc?key=0Au-5jsYGHc29dG94ekRvbEl1WEUxcnFYMU14eDRRV3c&usp=sharingThe 7970 is about $.06 more expensive per KHash/s and per watt.
7970's in a 3 card setup will take roughly 17 weeks and 1 day to break even where as the 7950's will take about 15 weeks and 3.5 days. Surprising that what seems like such a small difference actually has a pretty big effect? $.06 per MHash/s and per watt equates to 11 days difference in the break even time... 10% faster return on investment.