I want to swap my m770 3x10 XT cranks for 2x10. I want lighter and a narrower q-factor. I would like to keep my current XT shifters and FD if possible but i still want good performance. I am looking at XX or XO in 26/39 and m980 XTR in 26/38. The SRAM q-factor is listed at 166 i cannot find info on the XTR. My current XT is 175 q-factor The prices are similar between them except the XTR comes with a BB. What do you think my best option would be?

Be aware there are two XTR crank versions:
FC-M985 'Race' 2x10 only
FC-M980 'Trail' 3x10 and 2x10

The M985 has a narrower q-factor than the M980. Do a search the and figures are on here somewhere.
One important consideration with the M985 is that because of the low q-factor, the crank can clash with the chainstays of many frames. Do your research before buying. For instance my Cannondale Flash driveside chainstay has about a 2mm clearance with the M985 crank.

XX also has a narrow version which is 156mm IIRC.

What bike is it going on?

I have the X0 and I would recommend the XTR over it as the carbon really gets battered to the point that the very end of the arm starts 'flaking' bits of carbon. I've resorted to wrapping the ends in duct tape, similar to what Lightning cranks do with their rubber caps.

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Norwood & Adelaide Uni CC

If you do go with an XX or X0 cranks these beauties work very well and are very cheap:
http://raceface.com/components/small-parts/crank-boot/crank-boots/