Help! Osprey Xena 85 vs Osprey Ariel 75?

Hey crew, I have a job leading wilderness trips in Colorado for the summer and I need to get a new pack! I'm totally stuck between the Xena and Ariel, having ruled out all other packs (I just wish the Aura were larger, then my choice would be easy!).
I'm looking for opinions about why you (or your female companion) have chosen one pack over the other. I know they are both great and will both get the job done, I'm just curious to see how other people walk through this process since I can't commit to just one.
I am a small lady, 5'4", but strong and I will be carrying a lot of gear - personal and group - over some significant distances. Our longest outing is 8 days, so I want a pack that is comfortable, durable, and well-organized for medium to longer trips. My personal trips tend to be shorter, in the 3 day range, but I also want the option to be out for as long as I want with everything I need. As it is, I would rather have a larger pack with less stuff in it than a small pack with not enough room.
Thoughts?

Xena in Small is 81L at 5lb 3oz
Ariel in Small is 72L at 4lb 14oz

Hey! Did you ever choose a pack? I'm considering both of these as well and I'm torn between which I should get! Thanks!

Leading a group always go bigger. Just packing for yourself, always go smaller and make it work.