Well, prices are a consideration in booking guided tours, and we like to follow RS book and tour a bit on our own. But with SO much to learn and see in Rome, what tour is tops on your list to do if you could only do one with a guide and had to do all the rest on your own??? Vatican, Colleseum, etc. Thanks again for all your great advice and for sharing your personal experiences. Oh, we are traveling with our scholarly 9 year old, so keep that in consideration. Although she will probably outlast us with her attention span with good guide!!
The Vatican ... hands down! There is sooooooo much there, and things are not well marked; it's great to be guided through the "best of" areas.
The hop on and off bus tours. There are a few to choose from. It will give you a good overview of many major sites. We sat on the top and had the ear phones that told us in English what we were passing by. I got lots of great pictures too. See St. Peter's and take the elevator up to the inside of the dome. Incredible to look down on the floor of what you just saw. If you are all energetic, start the hike up the very top for incredible views of the city and the Vatican. If you can, try to be at the top of the Spanish Steps at sunset for the view of the sun setting over Vatican City.
The Vatican Museum, for sure -- to skip the line and get good info. Also . . .Make sure to buy the flip book that shows the ruins as they are today and as they were originally. You and your daughter will appreciate it at the collosseum, forum, and many other sites -- it really helps those places come alive, and there is minimal signage/interpretation at the sites. You can buy it at the stands near the colloseum and many other stores/gift shops. I'm not sure if you can get it in the States, but if so, it sounds like your daughter might be the kind of kid who would like to study up ahead of time so as to maximize the experience and make some choices about what she definitely wants to see.
The Colosseum & Palatine Hill. Remember that Rome once ruled the entire world, right from that location. Your walking in 2000 years of history, standing in the same places the ancient Romans did. A good guide brings it to life and helps you see it as it was, not just as a collection of rubble.
To me, at least, the Vatican tour was overly long and after a while just overload. We did it with guide but I'm not sure how much he really added to the impressions you could get on your own.