Don't forget the 4670K is two generations ahead, so the 2700K would have to overclock around 15% just to catch up. So a 4670K at 4.2GHz is as fast as a 2700K at 4.8GHz. At that speed, even if it could be achieved, would draw about twice the power of the 4670K. And not to mention, the Z87 motherboards have a lot of extra bells and whistles.
4670K for sure.