I actually like the 65th better than both. I've sold the 650s and selling the 500s right now. I'm keeping the 65th. Someone said the 500s gives more space around the instruments. I don't find that to be the case at all. The 65th kills in sound stage and air around the instruments. However, I do believe the 500s does extend a bit further in the treble area.
I vote AKG 65th Anniversaries