The Stratos 32 also has side pockets, which neither the Atmos 35 or Stratos 24 have. Also I would suggest that a top loading design is more weather proof than a zip opening pack. The AirCore back is not as 'scooped' as the AirSpeed on the Atmos.
There are 2 places for the axe handle on either the Ariel or Aether. Underneath the top side compression strap (just visible in the picture you mention) or underneath the wings of the straightjacket compression. Which works best depends upon the length/shape of the axe shaft and also what load you are carrying (full or half empty). So you will be fine, I'm sure.
I agree with you about the Atmos. It is 50 litres but the concave shape inside does mean it is more difficult to pack in a super efficient way, more difficult than the cylidrical shape of an Ariel 55. Also bear in mind that the 50 litres of the Atmos includes the zip side/back pockets. So that also eats into the main body storage area, although most people really like those pockets. I would say that the 'apparent' extra volume of the Ariel 55 is more like 10 litres bigger than the Atmos 50.
The Ariel 60 is lighter but also somewhat simpler, than the 75. I can understand why base compartment access is something your girlfriend would look for. You have a couple of options. 1. Special order an Ariel 75 from any Osprey dealer (we have them in stock). 2. Or buy a 60 which is more widely available in-store. I think you'd like the weight of the 60, but prefer the additional comfort/features of the 75.
I know what Bryan is getting at, he wants to see a pack that is customised for features (to save weight) rather than for fit. As a pack manufacturer that's a hard thing to do. You can strip features off a pack to save weight but that may not be what everyone wants. Yes, you can have a pack made for you (just like you can have a car made for you) but you'll pay more for less (if you see what I mean?). We (Osprey) have the Aether 60 (Ariel 60 womens) which is a great carry and weighs in at 1.5kg. Not the lightest pack on the market but pretty light and with a proper back/suspension system. Finally, our pack factory whilst not in the UK is about as far from sweat shop as you'd imagine. Great working conditions and very experienced sewing staff. I hope this is of interest.