Either is going to be incredibly fast, but I would only buy the "K" series if its going to be overclocked. If not, it's a waste of money and you should go for the non "K" version.
If you do plan to overclock, if you can get either of them to 4.3-4.5 GHZ they will push any game you can throw at them with a GTX 780 are pretty good frame rates.
In theory the 4670K is a slightly faster chip clock per clock. I hear alot of people report that the 4670K runs hotter though and is harder to overclock to higher levels. The other factor is not all individual chips are made the same. You might get a really good 3570K that could overclock to 4.6GHZ without a hitch, or you might get a 4670K that can't get past 4.3GHZ or vice versa.
In reality if you push either chip past 4.3ghz any difference would be minimal, if any.