Wait, Penryn is about to come out and those Q6700+ Quads will drop in price.
Q6600 is good for now, if you don't plan on upgrading for a few years, and don't want to wait.
I plan on doing a new system next year, but the Q6600's price was enough for me to do a minor upgrade this year.
I wouldn't pay the 500+ for the q6700+ knowing that in a few months they will definitely be cheaper or I could just get a Penryn then.
Also remember about the time Penryn comes out, November/December timeframe, that AMD is talking of releasing their Phenoms. If they come close to Penryn performance, oh what a choice!!
The price wars will continue.
Just depends on your current needs, can you wait another few months to upgrade or do you need it now.
If you need it now, easy answer Q6600
If you can wait, then wait to see the new prices after Penryn comes out.