In my opinion i wouldnt waste my money on XTR v-brakes. (if you are using them for trials use). I find they create a spongy feeling at the lever, and do not have the power to hold up for trials use.
furthermore i had some that i used on my front brake that snapped on the "2nd" arm bit, then i replaced it and it did the same on the second caliper i use.
if you are thinking about them for trials use then avoid like the plague IMO. A much better brake caliper would be something like the AVID digit 5 from TartyBikes, or this shimano one here