Both have IS, but only the A720 has manual controls. If all you want in a camera is looks and good pictures without the need to learn more about photography then get the IXUS 80.
If however you want to learn as you go along clicking photos, then go with the A720 IS and slowly start using manual controls to get the picture the way you want it to come out and not the way the camera wants it to come out.
Hope that helps. But either way, with Canons image quality and IS (Image stabilization), you will still get decent shots.