Some if not many people can ride several different sizes of bike comfortably. I'd go for the size on the lower end. But the real way to figure it out is to ride both size frames for 10 miles or better. Then you'll have a real good idea which is best. Doesn't have to be the exact tier level bike you want, but it does need to be the same brand and year model frame.
Parking lot trials don't work quite as well. You just don't get the same experience nor time in the saddle.
If you can't test ride the bike and you have to buy it there, then trust the shop sales person to do you right. They do after all want your repeat business... I hope. Maybe get all the shop people to give you an opinion.