I see almost zero logic in trading "up" from a 15-55 to a 15-85 - get the 85/1.8 instead. And trust me, you will never pull out the millimeter-calibrated ruler when you're shooting to "measure" the amount of overlap in your lenses - either they're a useful range for you, or you have to change lenses more than you'd like, or they just don't get into the range you want. I've shot with a 16-35 and 70-200 two-camera setup with a 50/1.8 in a pocket, and ignored the 50 all day.