Im looking to order an XX1 groupset from Bike 24, and they have 2 sets each with a different Q Factor (a QF 156 and a QF 168), can anyone advise how im supposed to workout which one is the most suitable for me?

Linky

It’s basically how much clearance the crank arms need to miss the chain-stays..

What’s the bike / current cranks?

Cheers Frogstomp, its to go on My 2011 Fuel Ex9, currently running a 3×10 XT setup.

9/10 it’s QF 168:

You need the 168. One of my mates has the same bike and recently confirmed with Trek that he needs the 168 Q-factor cranks for the XX1 chainset.