You could go either way, and it shouldn't be a problem.
One thing to consider is that the most typical joint size for the UniLoc is .842" in diameter.
The 5/16-14 is most typically .840", and the 3/8-10 is most typically .850". Although these are not cast in stone they are what most performance shaft companies use as the standards. So, if you went to the 5/16-14, and your cue's diameter is .842", then most 5/16-14 shafts will fit pretty good. But, if you went with the 3/8-10, then most shafts made to fit that joint would be larger in diameter than your cue is.
Just info for you to ponder!
Royce