There isn't much difference, the turbo boost potential for the 6500 is up to 3.6ghz, the 6600 up to 3.9ghz. A 300mhz difference. I'm not sure how much price difference you're looking at, depending on where you're from. In the u.s. it's around a $15 price difference, at that price I'd say it's worth it.
Prices shift, there's a $20 difference between the i5 4590 and 4690 (the previous 4th gen versions of those two cpu's). The price difference used to be closer to $30 between those and they only have a 200mhz difference which made the 4690 non k a bit overpriced for what it offered. With less cost difference with the two you listed and a 300mhz speed difference I think it changes that but that's my opinion and what I think is 'worth it'...