SANS 660 is being run for the first time at SANS London 2010, so I can't comment on the entire course, but I know some of the content has come from 709.
My take on the two courses are:
709 is intended for folks that want to find, created and develop their own exploits, or at least have a stellar understanding of how the bad guys and security researches do it.
660 is for those penetration testers who want to take it to the next level and understand, subvert and overcome the defences of modern systems and networks.
I really enjoyed SANS 709 and have a vastly deeper understanding of its topics from taking the course, but it's a very, very targeted group of people that really get the most of it. 660 sounds awesome and a lot less mental pain
My suggestion is that if you've taken SANS 560 then 660 is going to be the best course for you.