If you don't plan on overclocking then get the 4670k since it would be slightly faster even at the same speed but also allows you to have the new socket in case Intel brings out something better in the future that you would want to upgrade to.
Though, if you aren't overclocking, why the K series? The whole point of them is to be unlocked to allow overclocking. Maybe some day in the future instead of upgrading you can just clock it up to 4Ghz or so, which will seem like a small upgrade anyway . . . but if you aren't even going to do that there is no point in buying the more expensive K chips.