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what would you be using the system for?
if its for gaming i would go for the 6600K, AMD CPUs are quite out of date as they havent released anything for ages.
If you've got the budget for a 6600K and you're going to be gaming, it's got to be Intel.
I've thought the 7850K is pretty great, as the onboard graphics are pretty strong. From a CPU perspective it's a fair bit behind any recent Intel offering.