If you:


don't mind selling any EF-S lenses you have if you ever get a full-frame body in the future


and you:


don't mind not having coverage between 55 and 70mm (not a huge hole, IMO)


I would highly suggest getting the EF-S 17-55mm f/2.8 IS. It has L-series image quality by all accounts I've seen, plus it has the IS you want. It's the only lens that I have on my XSi, and I absolutely love it. If you ever do switch to a full-frame setup, this lens should be able to sell easily.


The 70-200mm f/4 IS is actually the next lens I want to get to have with my 17-55! a great pairing IMO.


Just my 2 cents,


alex