You just have to ride both and see what's comfortable. The smaller frame will require a longer stem and seatpost to fit, and may have greater saddle to bar drop (if there is any) that might be uncomfortable for commuting purposes. The larger frame might have a too long top tube, making it a stretch to reach the handlebars. If you're in between sizes the best thing to do would be to ride the bike in the two sizes you're in between and determine which one feels best to you.