Any of the Italy tours will be wonderful. Your consideration is the amount of time you have to take a tour. You really need to fly in at least the day before a tour begins so that you will have time to adjust to the time change and get over jet lag. Many people fly in the day before and many fly in a few days before so that they can see what you don't see on a tour and won't have time to see in your free time. Also some stay at least a day or more after a tour ends or go other places. If your just interested in a tour, then fly in at least a day before the tour. Do not fly in the day of the tour. It is just too tiring. I did that on my very first RS tour and that was a huge mistake. I had been to Europe four times before but still made a silly mistake.
The longer the tour, the more you see of Italy. However, you just need to base it on how many days you can afford to be away. Whatever you choose, will be wonderful and will see a lot. Also research where you will be going so that you know what you want to see in your free time. Do not wing it. Make a plan and a list of where you want to go. See what you can and if you don't get to everything, well that's okay. Just have a great time and enjoy what you see.
Read the Rick Steve tour scrapbooks of Italy. They are a wealth of information about the tour and where people went in their free time, what they brought with them, what mistakes they made, what tips they have, where they ate and just in general about the tour. Also their photos are wonderful.
As for comparing tour companies, I cannot say. I have only been on one other tour company tours years ago and that was for 18-35 year olds. But I will say, the RS tours only have 24-28 people on their tours and no tipping to the guides and all that is listed that you will see is not extra.
Hope this helps you making a decision.
Have a great trip.