Both will fit, just comes down to preference and/or if you get heel rub. I went with the 168q on my xx1 cranks, but I'm sure I'd be just as happy with the 156q. I wear a 46/47 shoe depending on the brand and found that I only rub the chain stay if I do it intentionally (I.e. when I was deciding between both q factors).
I've run narrow q factors on other bikes and only notice a difference when pedaling through ruts...even then it's barely noticeable. I have approx 13mm of clearance between the crank and chainstay...plenty of room for a narrow crank. Hope this helps.
Niner says either - I, too, put the 168 XX1 cranks on. I wear 46.5 shoes, but with all the pedal float never had an issue with hitting the chainstays. I opted for the wider setup because after doing a pro fit, it really smoothed out my pedal style and feels much more powerful.