Just found this on Quilter's World website. Is this what you are working on? If so, there are no points to worry about, so I would go with 12 1/2 square if you can.
Another thing to consider. Several times, I've ended up with blocks all close to the same size but smaller than the directions. Since this quilt has plain borders around it, if you happen to end up with your blocks only @ 12 inches, don't worry, they will all fit together and you'll just have to adjust your borders a little when you get to that point.
I may have this magazine at home, I'll take a look after work.