Actually, X99 has more lanes and I think even with basic 5820K CPU. On X99 you can actually run 2 graphic cards with PCIe x16 mode on both where Z170 can only do 2x PCIe x8 mode. I mean, even my ancient X58 can run 2x PCIe x16. Intel kinda reserved this to the highest end...
Skylake is certainly the way to go product node wise, however as things stand now, hexa cores sound more appealing.
Firstly, 6700k is actually a very crappy overclocker. Considering it's running at 4,2GHz and people brag about "extreme" overclocks and then they only get up to 4,7 GHz. If this is "extreme", then what's 5820K going from 3,3GHz to 4,5GHz? Also, with DX12, a lot of games displayed a huge benefit from more cores. So, even basic 5820K has 6 physical cores and 12 threads. Which is significant for today's "conditions". I mean, Skylake is still exactly the same configuration as prehistoric Core i7 920... Hell, even i7 980X was a hexa core and that was like 5+ years ago...
6700k caught my attention due to 16nm, but frankly, X99 with 5820K sounds like a better option...