right now x99 supports PCIe 2.0 x8 for its PCH, while Z170 supports PCIe 3.0 x20 PCH. I recommend waiting to see if anything changes for storage interface, which it likely will not. If that's the case, x99 may be the worst chipset in history, with key-m SSDs that are much much faster than current M.2 key-m technology (i.e. Sam's 950 pro, or intel's similar nvme) on the horizon.
I have been interested in x99 myself, up until I realized latest technology leaps in SSDs using NVMe will allow PCIe 3.0 x4 to saturate.
EDIT: It appears x99 uses DMI 2.0, a x4 pcie 2.0 interface limited to 2.0 GB/s, which is a bottleneck for a single 950 pro