Last summer we did tours of both the Vatican/St.Peters/Sistine Chapel and Colosseum/Forum. We got the most value out of the Vatican tour. The Vatican Museum is a zoo and it is very easy to get lost in the crowds and miss the most important works without someone to help navigate. Our guide was outstanding and brought to life both the history and the modern day role of the Vatican. If you can get the early entry tour, I would recommend that. We took the tour in the mid-morning as this was the only time we had available. If had to do it again, would have opted for the early tour.
The Collosseum/Forum we could have done on our own with the RS guides. If you go to the Colosseum do so early in the morning. It gets crowded and the Forum is hot in the afternoon. Bring plenty of water as there is no place to buy water in the Forum except and expensive vending machine.
Have a great trip.
Sandy