When my wife, and I decided to go on a Med cruise 2 years ago, I had a full bag of all sorts of "Kit" lenses for my Canon 50D. I decided to sell the "kit" stuff and buy just one lens for a walk-around-lens for the trip. I looked at Canon, but they just didn't have the range to make use of just one lens. I decided on the Tamron 18-270mm with VC. It worked just fine except the 18mm end was not wide enough. It seems while in Rome, the streets and ped. malls were to narrow, and the 18mm end was not wide enough to get the entire shot. Also, the fountains and churches seem to be crowded together with tourist stores etc, on both sides of the little streets. So, I would definitely take a ultra wide angle lens on that 40D, like a 10-22mm. At the forums or larger openings use a mid range zoom on your 5D. Remember in Rome, most all the outside building are white marble, with marble pavers, so a good ND filter set is a good idea for those bright days at noon or later. Be prepared for crowds, everywhere. In St. Peters, there is very little light, and no flash, so take lenses that have a good low light ability. 2.8 is OK, but your 50mm 1.8 is the best in this church. Be prepared to stand in line at the Vatican, take a liter of water with you in a backpack, with your lens selection for the trip. After the Vatican, you may want to swing to the Collusium, it will also take hours to get through it. You will need the self-guided tour phone. Here you will need the mid-range zoom, and the Ultra wide angle. Outside of the Collusium is some of the oldest ruins in Rome, so take a walk around and see the old pagan temples. Have a great trip, and be ready to do lots of PP.