zacotton,


First, full disclosure, I have had the 17-55 since January 2008 and it is my favorite lens on my 50D. I have taken thousands of great pictures with it . To me it is worth the price difference. The main advantage for me is the f/2.8 aperture. The IS is also very nice to have for me as well, but it is secondary to the aperture.


If you do not need to let in twice the light or the IS, the 17-40 is a great lens. The other thing you would be giving up is a little more reach. This should be easy for you to decide if it is something you need or not. Look at your images and check how many you took between 40-55mm with your 18-55. If it is a significant number, then you need to go with the 17-55 for the upgrade. If most of your images are from 18-40mm and you do not want the extra 15mm reach in the future, you will be happy with the 17-40. Although you may gain a little light with the constant f/4, but you will lose IS, if that is important to you.


I hope this helps,


Chris