OM is exactly right: fit is everything. You'll be able to find the capacity you want in almost any pack line, but each brand's suspension (and sometimes each model line's within brands) will fit differently. Go for fit first.

Osprey is a good brand, but don't overlook the Deuter lines, either, particularly the Aircontact and ACT Lite seris. If the Ospry isn't comfortable, the ACT might be.