You mentioned in your original post that you will eventually upgrade your camera to a full frame and in your latest post you said that you rarely zoom past 100mm. The 17-55mm lens is a great lens but it won't fit the full frame mounts. It only fits EF-S mounts like your 50D. The 70-200mm lens is one of the best lenses in Canon's line-up. If you don't zoom past 100mm that often, you should consider the EF 24-105mm f/4 L IS USM Lens. It provided 3-stops of IS and is a great walking around lens. It's not very heavy at all. The image quality of the 24-105mm lens and the 24-70mm lens are pretty much the same. I think that this is one of Canon's best.