I won't tell you which focal length you should use, but if you go wide, I definitely agree with those who suggest an EF-s lens rather than a wide EF lens. When you buy a lens like the 16-35, you're paying a premium (in weight and often in quality as well as dollars) for a lens that can cover the whole 36mm frame. In other words, similar EF-s lens will be cheaper, lighter, and probably better.