What I'm about to say might be bad advice. Maybe somebody with bike fitting experience can weigh in on it.
If the bike comes stock with a significant amount of setback on the seatpost (like the kind of seatpost you see on a stock Surly LHT), get the larger frame. If you find that it's slightly too large, remove the stock seatpost and install a straight seatpost. The double bolt style seatposts Thompson makes are a good example of this. You can also swap the stem out for a shorter one to bring the handlebars in a little closer.
If it comes stock with a straight seatpost, get the smaller frame. If it's slightly too small, then swap the seatpost out for one which has a lot of setback, and possibly also swap the stem out for a longer one to move the handlebars a little farther forward.
Again, this might be bad advice and perhaps someone with bike fitting experience should weigh in on it.
You could also just go to a bike fitter. It may be worth the money, and then you'll be able to better determine exactly which size frame would be the best fit for you.