I, too, am looking for a smallish backpack, and yesterday looked at a bunch of them in a Best Buy store. The SlingShot looked great, but it's designed -- as most of the "sling" type packs appear to be -- contoured and designed for right-handed wearers. Only "fits" properly on one side of the body (and not the side I'd want it on). There was a Manfrotto (I forget the model) that looked like it might work, but it was just too, too pricey.
UPDATE: more research, turns out Kata makes several "3N1" bags (different sizes) that can be worn as a backpack or sling-pack on either side by changing the configuration of the strap. Checked them out online and at a local shop. The shop only had the largest size (too big), and the clerk said he thought the Kata bags would be going away soon. B&H have them at a very reasonable price (under $50). So I just ordered two, one for each of us. Will report back after we've knocked them around a little.