Both will do the job equally well. And because of the massive market in the States for civilian AR15s, there are probably just as many butt stocks available in mil-spec as there are commercial size. In fact, some companies (magpul for one) offer the same butts in a mil-spec OR commercial configuration. Personally, I would buy mil-spec. In my experience the butts are usually snugger to the buffer tube, and I don't like a sloppy fit.
Cheers.
Tim H