To quell further debate about the specifics - we will take the morning tour to the Vatican museums and buy a normal ticket for the Colosseum and book in advance if we can. The Colosseum tours get you onto the arena floor, and lower ground areas, but really once you have seen one dungeon you have seen them all. It is cheaper during the day, which also includes the Forum.
The only time I have been to the Vatican museums was in June 1998, and the Sistine was packed to the gills then. The queue for the Colosseum was not that bad, but I anticipate that it will wrap around these days. You could argue that it is more impressive from the outside, but since we want to go the the Forum/Palatine I figure we should pop in to the Colo as well. And get the photo with the "gladiators" outside.
