I voted "as stable" but it's a bit early to have a good feedback of 6657+. 6657+ seems more stable though I had few problems with 6510 which were properly recovered. I'm not using 6510 anymore.
I voted 6.6.5.7+ is somewhat less stable because in 3 days of works i have had 5 crash to the desktop. (in the latest crash i had 2 error in a row #30009 and after the #91 but only one time each) Now i know the trick to avoid that type of crash so i can work again without big trouble