I don't get putting a X99 MoBo in a "gaming rig". The only reason to do so, from a performance standpoint, would be to use 3 or more GFX cards ...and with SLI no longer officially supported with 3 or more cards, unless that box is going to serve a dual roles as a rendering or video editing box, then I would tell you that, while the split wins in gaming test Z170 wins more than it loses up against X99.... at Z270 is faster than Z170. And, if anything and tho I cringe using the word, Z270 will be more "future proof" than X99.
As for choosing between the two, I am not a fan of either ... on newegg, more board owners are extremely disappointed with the board (1 egg reviews) than those who love it (5 eggs).
If it's me ... well again, if it's me Id build a Z270 box in late April after the bugs wash out ... but for X99 I'd choose the:
MSI X99A XPower Gaming Titanium
MSI X99A Gaming Pro Carbon
MSI X99A SLI Krait Edition
You can return unopened packages no problem.
When doing a build, picking the most expensive item in each category, usually will not deliver top performance. The Titan for example will not get you gaming performance as good as the x80Ti.