If you get the 6600k you will get increased performance in terms of frame stability. Less drops less often and when it drops it won't be as massive. Thats just from the updated micro architecture. That said i would almost say the single core performance would be better on the 6600k. Also like rockyplays says. Later on if you so choose to upgrade to cannon lake z170 platform is compatible with the newer i5 not the older i7.
But you have to way the cons as well you're not going to get those 4 extra hyper threads so if you plan on streaming or Photoshop rendering the older i7 should be faster. Though even then i have doubts as the i5 has newer microarch.