Judy, a rail pass is rarely the best deal when traveling by Trenitalia train in Italy. I say Trenitalia because Italo, a competitor, does not accept the pass. That being said, a lot of the decision depends on how much work and planning you wish to do to obtain the nicely discounted tickets that are less expensive that those offered by Eurail. Plus, in Italy, you pay a 10Euro supplemental fee for each fast train you ride in Italy. You must make advance reservations on the Eurocity and Freccia trains that you will mostly be riding. That could be 4 such trains to/from Milan/Chiusa. It's possibly another 4 trains to/from Chiusa/Rome/Lake Como. That's an extra 80Euro on top of the cost of your pass. A Eurail pass will get you onto any regionale train at no extra cost. Your trip to/from Lake Como will be on regionale trains. Most likely, you might also be on one to/from Bolzano to Chiusa.
You don't indicate when you will be traveling so it's difficult to quote fares. However, if you can book 90 days in advance, you can save up to 50% on the fares from Milan to Bolzano (24Euro vs 50Euro), Bolzano to Rome (62Euro vs 89Euro) and Rome back to Milan (29Euro vs 76Euro) on the way to Lake Como. The discounted fares are non-refundable. If you don't want to lock in discounted tickets on specific trains, you can purchase the rail pass and have more flexibility in your travel. You will pay more overall for the pass.