In my case I went to a local Moosejaw with all intentions of getting the Aether 85. It is only 1-2 oz more in weight, but has 15L of more space in it. Speaking of which... how do they figure volume so far as what can I visualize getting into 15L of space in a backpack? I know it isn't 7 2-liter bottles of pop although that would equate to ~15 L
I checked with a number of places and in my case I used something like Philmont as the 'baseline' for what I might need as I would probably never do a bigger backpacking adventure than that... However, I know that they refuel (more food, etc.) every 4 days, and a lot of people actually suggested not going bigger than the Aether 60L!
I think the 2-3 problems I had were
1. Buyers remorse... any time you spend that much money on something you want to make sure you got the right thing.
2. The 85L is only an oz or so more and only about $20 more but seems to offer a ton more space.
3. I probably tend to be like most novices and pack too much
Reality is the person at the store convinced me to go with the 70L for the very fact that he said I should have no problem packing enough for 7 days... If you can squeeze 10 days out of it that would be even better, but I don't plan on going 10 days anywhere where I have to have everything for the entire length of time...
As a side note, from another local high end camping store, I got 'free' (quite nice) an osprey day pack that says it is a 20L pack. Just for being my birthday month. Seems extremely tiny in comparison, but someone said I could always strap a day pack on the back of the Aether 70 if I needed to for 'extra' stuff... my logical thought would be get a pack that is big enough in the first place versus bailing wire/duct taping a solution. I found some boy scout sites with recommendations and a lot of them steered towards the 65L pack although the Philmont web site says they recommend 75L size as a minimum internal pack size.
I have a long ways to go to get experienced with backpacking, but looking forward to it. Part of me still thinks the 85L would be nice with no worries on space, but all of the more experienced people have basically said that it was WAY too big for what I need.