Easy decision. Go with the 28 IS.
New Canon lenses have better/updated design with new coating formula that results in much much better image quality. Yes, you have wider f-stop and smaller DOF when using f/1.8 of the old 28/1.8. But it's not that useful when the image is soft wide open and you'll need to stop down to see sharpness.
there is a trade off in shutter speed in lower light conditions