Stephen Yutzy wrote an answer about Osprey Packs Aether 60 Backpack - 3500-3900cu in on March 19, 2012
As Phil and Thomas said, if it's at all possible get yourself to a store, get fitted, and try them on. That will probably answer your question better than anyone online could. You might also find a 3rd or 4th option that would be even better for your intended use.
For mountaineering use, keep in mind that you'll have extra gear requirements vs regular backpacking, and different packs have different systems in place for handling that gear (external tool attachment points, ways to handle crampons, floating lids that can accomodate a rope underneath, etc)