Thanks for the suggestions for Rome. I was inclined towards the 5D and this has confirmed it. I wonder whether POTN has any members in Rome, or Italy anywhere?
I'd kind of recommend you even leave the 24-105L at home, you won't really need anything more than a 50mm... and the Sigma 12-24mm is simply a ridiculously wide fun lens, and for that reason alone I'd recommend you take that as well.
People tend to bring too much gear with them, especially on trips, and it's actually detrimental to the overall quality of the photographs they make on said trip, especially if they mainly use zoom lenses (which simply make one lazy).