Sign up for tours of main sites in Rome or use Rick Steves guide book and/or audio tours?

Depends on how you like to travel. If you want to sit and walk on your own schedule, the audio tours (or book) are great, I don't like to be herded along on a tour guide's schedule, so I had Rick in my ear or in my book. Other people swear by the live tour guide. Or you could do both -- try one and then the other for the next site, to figure our which works best for you.

The big advantage to live tour guides is that they can answer questions about this you don't understand or want more information. We think they are always worth it.

I find that a really good guide always enhances my enjoyment. However I find it's really important to choose carefully… A large group tour makes really gets in the way, in my experience. I really like Context Rome. They have very small groups, no more than 6, and remarkably well educated guides. I had been to the Vatican four times before I took their tour, yet I learned so much from them. I also find a good guide to be really helpful in archaeological sites like the Forum.

I've always used Rick Steve's podcasts. I spent an entire full day in vatican using them. At my own pace. Best way to go

It can really depend. I have had a guide and I have also gone on my own for all the above listed sights. In general, I prefer the audio guide for crowded sights e.g. Vatican. However, the Vatican with a guide could save you time waiting in line. Or book a reservation and then use the audio guide. I like to listen while I'm looking vs reading and looking up and then down. A guide can make the forum come to life. But for Palatine Hill, Coliseum and St Peters I don't think you would benefit from a guide. The audio tours are great for those places.

Thank you all for your help! I think I will probably go the route of Rick's audio tours at least for the Vatican.