I'm fairly new to backpacking and am purchasing my first pack! Up until this point I've used hand-me down packs from much larger friends so I'm am new to this whole 'backpack fitting properly' thing.

I am looking at the Osprey Women's Aura (65) or Ariel (55). According to their measurement tool, I have a back length of 18 inches, which puts me at the large end of a Small (16-19in) and the small end of a Medium (18-21in). Any wisdom as to which would be better?

I have found that the medium size restricts my head from moving back at all (because the back flat part comes much higher than the small), is this a concern?

Alternatively, if anyone has any input on the Aura vs Ariel I'd love to hear it! I've found the aura to be very comfy on my back and I have a lot of freedom to move while wearing it, but it does feel like its pulling me back in the shoulders a bit. Is this a normal effect of the antigravity suspension or would this cause discomfort when wearing it for longer trips/heavier loads?
The ariel is a bit more restrictive but it doesn't seem to pull backwards on my shoulders as much and the hip belt seems a bit more comfortable.
Thoughts?

Sorry for rapid fire questions, but I figured this would be a great place to get advice! Thank you in advance!

One question: what kind of load will you mostly be carrying? The weight and the bulk of the load are a big part of pack fit. If the load exceeds the carrying capacity of the pack (and Osprey is good about indicating comfort range of its packs on the website), it means the suspension can be overwhelmed - and everything else is irrelevant. Likewise, there's no need to carry a pack that is built for heavy loads, if you'll never exceed 25 pounds - the pack will be overbuilt for what you're doing, and will be a pound or more heavier than it needs to be.

As far as the Aura - make sure the person fitting it to you knows how to do it. When I tried the Atmos AG (men's version of Aura AG) on at REI, it felt like it was pulling me backwards, too - and their pack fitting "expert" never could get it right. When I tried it out at my local shop, the clerk there understood how the pack worked (essentially, you have to move the yoke a bit when you move the load lifters, so they'll work right.) It felt great, and the pulling-away effect disappeared. I'm also at the top end of a medium and low end of a Large, and found the Large to be the better fit.

Finally, don't let them fit the pack with sandbags for weight, unless they do it with 5 pound bags (or maybe a 10), and use foam-filled bags or bubble wrap to keep the weight where it would be in the field. If they just dump 20 pounds of bags in the bottom, you'll never be able to tell how it fits; the weight isn't distributed correctly. Better yet, take your full load of gear (including a quart or two of water and the amount of food you'd carry for a typical trip) into the store with you, and load the pack normally. This not only tells you how it feels with your stuff, it will let you find out whether the pack is way too large (or small), and will reveal any loading issues (for example, whether a bear can will fit.)

I've used the Aether (men's version of the Ariel), and found it to be extremely comfortable and total overkill as I reduced my pack weight below 25 pounds. Now that I carry about 15 pounds of gear, food, and water for a two-night trip, I'm much happier with the Osprey Kestrel 48 (Kyte, for women) - which is nearly a pound lighter than my Atmos. The Atmos is still a great pack, and I intend to use it for longer trips and winter loads, when both the weight and the bulk of the load increases by 5 - 10 pounds. (I could save another pound by switching to the Talon, but I find that I prefer a bit more suspension than it has.)

After taking your stuff to the store, select the two best fits. Then buy both and try them out at home with your own stuff, and various food loads (say, a 3-day trip, a week trip and a 10-day trip). Play with the adjustments. There is good information on the internet to help you with this. Then after several days walking around inside your house, you may be able to decide which is better. Return the other. Just keep all tags on the packs and be careful not to get them dirty.

This may not apply to you, but if you plan on backpacking where a bear canister is required, be sure to see how that will work too. Most smaller women's packs are terrible at accommodating a bear can.