For me, this is an easy one, given your question: "Let's say you were traveling to a new country and wanted to be ready to
capture all the sights, which lens would you rather add to my existing
bag?" A few weeks before heading to England a while back, I made the 'mistake' of renting a 70-200 f/2.8L IS to use with my Rebel XT. It provided such a huge image quality bump over my 28-135 that I pored over Bryan's reviews for a solid week and then bought a 24-105 f/4L IS for my trip. It was a great decision, and I have loved that lens ever since. I knew that I wanted to head to full frame some day, and after a stint with a 40D, my lens is now on a 5DII. You can always sell gear (ie get a 17-55 f/2.8 EF-S for now and sell it when moving to FF), but I wanted to avoid that hassle. The IS and smaller size/lighter weight of the 24-105 did it for me over the 24-70 f/2.8L. To each his/her own, of course, but those were the deciding factors for me. All of that said, I should add that the 70-200 f/4L IS is a spectacular lens, and I have a newfound appreciation for it as a landscape lens. But given all that you've said, I say go for the 24-105.



PS -- Even on a 1.6x body, the 24-105 f/4 can still provide a nicely blurred background, given the right conditions, of course: