well do you find yourself using IS a lot on the sigma? If not, then just go with the non-IS. and good luck with your lens addiction, it happens to the best of us hahaha
btw, the 28mm is a pretty good lens. i have the f/2.8. It's nice and sharp. some people say it's an awkward focal length, but i haven't had any problems with it, i think they were on full frame tho. this photo was taken with it, i actually think this looks sharper full size.