The fit difference you're feeling (part of you, versus not part of you) is because you're comparing apples to oranges. The Deuter ACT line are true internal frame packs; the Osprey Aura is an external frame pack (the giveaway is the mesh backband (also called a "trampoline suspension.") Neither type is inherently superior to the other, but the feeling you're describing is the classic difference between internal and external: an internal frame pack hugs you, and the external fights you (the new breed, exemplified by the Aura, is a huge improvement over the old-style.) I agree with Aimless - you won't be doing acrobatics, or even very much exaggerated twists and spins, so the hipbelt fit is probably more critical than the hugging.

Just to see if you can have the best of both worlds, go back to the store and try an Osprey internal frame pack (the Volt might be a good choice - however, it's a unisex pack, so the harness may give you problems.)

I've used the ACT Lite and ACT Zero (same suspension, bag and feature set is different), and liked them both. You really can't go wrong with either of the packs you selected.