The best thing to do if you have not tried it already is go into a store and try both packs, loaded up with the same weight and see how they feel. I have the Aether 85 and its the best pack I've ever had. My next pack will be the 70.
Here are the reasons that the Aether is superior to the Atmos:
1) Comfort due to proper load transfer. The reason for this is because the Aether has a superior hipbelt and frame. Also on the Aether you have two straps on each side of the hipbelt to secure the pack load to the belt(the atmos does not have this), which secures the load to your body, making it more stable, thus giving better load tranfer to the lower parts of your body, thus feeling like a lighter load than the Atmos.
2) You have a removeable lid for a day pack, or shave almost a pound and leave it behind. I personally like a lid for the organization.
3) Aether has better load carrying ability.
4) Aether has the ability to ajust the torso, and or swap harness and belt for a different size. I have a large pack but swapped my brother in law for a medium hip belt, and it fits perfect.
As far as I am concerned the Aether is the best pack on the market right now, but you have to find out what works for you.