hey gabriel_p,
I would go X99 on this one. The previous posts did mention some great points though, particularly about gaming. There would be no real difference. If you REALLY wanted to split the hair you could say the 6700 would perform better in games because of its slightly faster core clock which.... again we are splitting hairs here.
I am personally running a i7-5930k and I do a lot of 3D modeling, animation, rendering, video editing, and motion graphics and I can tell you from experience that more cores is what you want even though they have lower clock speeds. Programs like Maya and Aftereffects will leverage the extra CPU cores and be able to finish tasks much quicker. So even though the skylake CPU is technically faster, it is still a quad core. The 5820k is only 0.4Ghz slower but offers you 2 more physical cores (and remember with hyperthreading that's 2 cores 4 threads).
Hope this helps!
P.S. I also like my game time and I am not hurting for FPS 😉