This may be too late but...I just got off the Rotterdam last
Friday, and went on that very shore excursion. You only get to see the outside of the Colusseum, and the outside of other monuments, but to get inside the Colusseum in the summer could easily be a two hour wait. Even the "Footsteps of the Emperors" went inside the Forum, but not the Colusseum. This tour went to the Vatican, and inside Saint Peters, and did an Express tour of the Vatican museums, including the Sistine Chapel, for HAL tours after the museums closed. This express tour was about 90 minutes, so you aren't wandering the magnificent halls for hours, but you do get in. Again, the lines for the Vatican museums in the summer are also very long, and could easily take hours. The frustrating part of this shore excursion is the guide gets to edit what you are going to look at. If I were the guide I would have given free time inside Saint Peter's to look around, but he didn't allow for that. We spent a bit of time on a bus tour of Rome that I'm not sure was necessary, but maybe we needed to take that very route that seemed to pass the same mermaid fountain 4 times. If you take the "transfer" excursion, that drops you at the Spanish Steps and picks you up there you will have less time in Rome, as that tour wants to be back to the ship by 6. You could probably only get to either the Colusseum or a short Vatican visit, and either would scare me, as I would worry about missing the coach due to subways running late or something.
The lunch included with this excursion was absolutely wonderful. In my previous visit to Rome I didn't stumble into this type of "typical" Italian trattoria, and I would certainly be happy to return to this one.
Good luck in what ever you pick. Rome was absolutely beautiful.