I've been to the Vatican three times, never with a guide. I did take 2 art history classes, whatever that is worth.
I find guide groups to be slow and boring. I would recommend reading up about it the day(s) before and maybe getting an audio guide that will allow you to go at your own pace. A lot of people even bring books along with them. It takes ~45 minutes to get from the entrance to the Chapel when hustling, I can't imagine how long a tour guide takes.
Just my 2 cents. I love Roma, have an awesome time!
We've done the Scavi Tour.... Probably the best "tour" we've ever been on - and we are not tour kind of folks. It takes a little over an hour, cost about 10 bucks, and was great... though they do not allow photography down there.
Google "scavi tour" and you can find a link in which to make reservations. I've heard the Vatican's response to the emails sent requesting reservations can be really spotty, so be persistent and keep emailing until you hear something. Also, this is something to do well in advance as they only take a few people down at a time (and you'll want the English tour). We knew a guy who worked at the Vatican and arranged it for us, so we were lucky. It is cool when you show the Swiss guards your papers and stroll through the gates... and the mass of people outside San Pietro are looking at you think you're something special
Good luck. Email or PM if you have any questions.
And visit the Vatican at 7am when the doors open. You'll have the place to yourself, and it is pretty special when you are there alone in the silence (not with the crowds of tourist).
I don't know how a $400 tour of the Vatican could be "worth it" unless you got meet the Pope or go into areas that are closed to the general public.
I took a tour with Rome Walks 10 years ago this October. It was a morning tour and then when we broke for lunch, the tour guide came with 5 of us to eat. (I think she liked me even though she was married and about 15+ years older. )
After lunch we met at the front of the Vatican museum, the lady gave us some tips, and then sent us in.
I really enjoyed it. I have studied some of that stuff, but I always think that tours are worth it. (Well not $400 tours.) You get so much out of your time with a tour guide that you would never normally get. Also, when walking from one location to the next...
**STAY NEAR YOUR GUIDE!! You will learn things that will never be told to the whole group.**
That's actually a good rule for all tours guides/ locals. ALWAYS stick near people who know the location, city, history... They have enough information in them for an 8-15 hour tour. Going to lunch with the tour guide gave us insights that the books and regular tours couldn't offer.
CT and Amalfi... been to both a few times. We prefer the Cinque Terre (we stay with Edi in Riomaggiore) over Amalfi, but both are good and it's just a personal preference. If we had the money to spend in Amalfi, it would be great, but our budget does not last half as long there. The CT has what we enjoy - hiking, great views, good wine and fresh pesto and fish... in a more laid back setting - not the ritz and glamour of Amalfi/Positano.
Again, i think it is a personal preference. I'd go to both again given the chance... just want to avoid driving from Napoli to Sorrento and Amalfi... yikes!