Such a hard, but fun decision!
The ABD that goes to Rome does it very well, but not much better than you could cobble together on your own. !
I couldn't agree with this more...at it's even closer than just "cobble together". I read the trip reports here very closely and looked at the itineraries...my son and I did just about everything in Rome that ABD does. And, on some things, our trip might have been a little better! Walks of Italy (WoI) does an outstanding Colosseum tour that goes to all three levels of the structure (something that, according to the recent trip reports here, ABD does not do). We also did a walking tour of Rome with WoI, and we visited the Vatican with them. We did not do a "private" tour of St. Peter's Basilica (although we could have), but we did climb up to the top of the dome. We also did a pasta making class with WoI that was one of the highlights of our trip (and very much reminiscent of the types of special experiences ABD often includes in their tours).
I put the word "private" in quotes because here is the thing...the word "private" is used VERY loosely when it comes to describing these tours (and not just by ABD). I haven't gotten a clear picture on the after hours Sistine Chapel tour that ABD includes, but I do know that they aren't the only tour group that gets "after hours" access. Just like Walks of Italy isn't the only company that gets "early entry". Which means, while the place might not be as crowded, you're not guaranteed to be standing there all by yourself. And, to be nitpicky, there is no such thing as a "private" tour of the Colosseum or the Roman Forum...there will ALWAYS be other people around you. What's "private" about it is you do get your own (very good) guide, and you may get some "special access" (like us going to the very top and the underground of the Colosseum with Walks of Italy), but you're not going to have the place all to yourself. We had a similar experience at Neuschwanstein on our Germany ABD last year. That tour was about as far from private as they come (Other non-ABD guests were put in our tour group, and we were so crowded that it really detracted from the experience. I wrote to ABD last year and complained about it).
We've done two ABDs, and to me, what ABD does best are the "off the beaten path" experiences. I think a big part of this is because the major tourist attractions of Europe have become so unbelievably crowded that you will pretty much always be experiencing them in a sea of humanity. But, ABD does a fantastic job of finding unique experiences that you might not otherwise ever hear about (some of them are even exclusive to ABD). This makes ABD trips wonderful for two reasons: 1) you get to see/experience things that you would otherwise probably never know about, and 2) it gives a nice reprieve from the hustle and bustle and crowds of the must-see attractions. All of this together makes for a very well-balanced vacation.
My point in writing all of this is to encourage you to think outside the box a little when planning your vacation. There is a lot you can do yourself on pre/post days, if you do some homework ahead of time. Will it be as convenient as ABD (with them handling all of your luggage, transportation, food, etc.)? Probably not, but if you want to maximize your vacation time and money, there are a lot of options out there to help you do that.