Both are really good. I would go with the 2500K because it can be easily overclocked and so can the 7850. Are those two setups equal in price? Is that why you're asking?
What motherboard are you putting these CPUs in? If it supports overclocking then definately go with the 2500K.
If you don't have any interest in tweaking or overclocking the 3470 and 660 are also great for gaming.