I have had both and recently sold both. Not because I didn't like either one, as they are both very fine lenses. The 17-55 was great on my 40D for indoor work. The 17-40 was great on my 5D for outdoor work. However because I also own the 24-105 4L IS I was using that lens on the 5D more and more because of it's versatility, range, and IS. My 40D has become my second camera used for indoor and outdoor sporting events with the 70-200 f/2.8L IS lens. Using the 5D more and more as a walk around lens I thought I actually had more lenses than I really needed. Both the 17-40 and 17-55 held resale value very well especially since Canon's price increase several moths ago. I sold them for darn near what I paid for them new 3-4 years ago. I think you need to decide what you shoot the most and let that determine what lens you need. At this point if I had to pick one it would be the 17-55 but only because I have the 40D and reserving the 5D for lenses designed for FF bodies. As a side note I found the color of the 17-40 a bit contrasty for my liking.