Do you already have most of the rest of your gear (sleeping bag, tent, and kitchen, especially?) If so, don't let the shop put weight in your pack: they'll do it with sandbags or similar objects, which will settle the whole load on your hips. That's fine for rough fitting, but not for making a final decision.
To get the fit really dialed in, take all your gear (including the amount of water you typically carry, plus enough food for the typical trip you'll be taking) and load it into the pack. This not only lets you figure out how to load the pack, but also gives you a true idea of how your pack will actually ride when you load it up.
I just went through this process when I bought my Aether 60, and it was well worth hauling everything in and spending 2 hours dialing in the pack. We started out with the sandbags in both a large and medium, to figure out the frame size, and chose a medium based on how it felt with the sandbags. Then, I loaded the medium pack with my own gear, and tried it with both a large and medium shoulder harness. It never did quite feel right, so we went through it a second time with a large frame, and ended up with exactly the right fit. Had we gone only with the sandbags, I'd have bought the wrong size.
If you don't have all your gear, maybe the store will let you borrow some of theirs to fill out your load. Or, at least, get them to let you put a sleeping bag or solo tent in the bottom of the pack, then put the sandbags on top of it - it might give you a slightly better idea of how the pack will truly ride.