Τhey are 2 total different sockets,x99 is the extreme platform and it has 6cores+ cpu's and the z170 is the high end mainstream platform and supports overclock on 4core and 4core8thread cpu's.
So its just what you intend to do with your pc.If you want to render videos and run virtual machines also anything that demands many threads x99 is the best option.In contrary z170 with cpu's such as 6600k and 7600k is much cheaper and better for gaming.