The difference between the two is the i7 has hyperthreading, an extra 2MB of L3 cache, and an extra 500MHz base clock (4.0GHz vs. 3.5GHz). Both CPUs have roughly the same overclocking potential and tend to cap out at around 4.6GHz unless you luck out on the silicon lottery. Up until recently, the i7's extras haven't been all that beneficial for gaming and the i5s offered much better value, as you could put the $100 you saved towards a better graphics card or an SSD or something like that.
Some newer titles are starting to benefit from the i7's extra threads, though whether that is going to benefit you is going to depend on your display's refresh rate and resolution. If you have a 144Hz monitor, the i7 could certainly benefit you, especially if it's a 1080p display where the GTX 1070 can deliver extremely high framerates so long as the CPU can keep up. If you're running 1440p or higher, then the benefits are going to be less apparent as the GTX 1070 tends to cap out at around 60 to 80 FPS at that resolution depending on the game and settings used. If you plan on getting a high refresh rate monitor, or want your system to last a bit longer and the extra $100 doesn't mean much to you, get the i7 6700k, or maybe look at the soon to be released 7700k if it's the same price. If not, the 6600k is quite capable, and the upcoming 7600k would also work well.
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