Single thread performance is close between the two. If you have apps that can use the extra cores offered by the i7 then the choice is simple. The 'K' version of the i7 has even higher base/turbo clock speed for best performance. Here's a comparison- Passmark Comparison
I7 hands down. Regardless of single core speed. Games are starting to move towards wanting more than 4 threads. So if you're on that platform, the i7 will last longest.