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!