A 2.8 lens is 1-2 ish stops faster than a 4.5-5.6 lens.
Image stabilization generally gives 1-2 ish stops back to you, but only for camera motion (or subject motion when panning in some models).
Thus, all other things equal, a 2.8 lens is pretty much objectively better than a 4.5-5.6 lens with image stabilization, because both of them wide open will shoot at the same speed for still subjects, and the 2.8 will do much better for a wider variety (or all) moving subjects. Also... more DOF control in your hands.
Edit: oh okay, this thread is over a year old, other than the update from the OP just now. But still. Above thoughts are relevant to many situations.