How heavy is too heavy for a pet vacuum if I have stairs?

Most reviewers consider anything under about 14 pounds 'lightweight' for a full-size upright, and lift-away or detachable-canister designs are strongly preferred for stairs. The Shark Navigator Lift-Away NV356E and Kenmore Intuition both come up repeatedly in stair-cleaning discussions because the canister detaches and the wand can reach risers without dragging the full body up the staircase.