verty, just put the L designation aside because BOTH lenses are hot performers. Now, having neutralized THAT debate with yourself, merely ask which lens has a focal length range that is MORE USEFUL for the camera body that you own NOW. If it is still a total tie, ask yourself if you will be playing the part of tourist on vacation, and which one you want to spend wandering around streets on foot all day...the 1.5 lb. lens or the 2 lb. lens. If you prefer saving 0.5 lbs. on your shoulder all day, pick the 17-55. If you never will travel as a tourist with the lens, and would always be carrying the lens as a paid beast of burden (a pro) then it is a total tie once more. Then ask yourself if IS would help to to take photos in low light; if so, the 17-55 is the lens of choice. Finally, if it is a tie, then bow to the L god and accept the fact that all those around you who own Canon cameras will bow as well (or think you are spending money too lavishly on a hobby)