I can elaborate a little bit on Dick's post, but mostly about food.
We stayed at the Sonya last year on a RS tour, and it was fine. I prefer the service and ambience at the Aberdeen, but we had no complaints about the Sonya. And I feel compelled to add that the breakfast buffet at Sonya included the best mortadella I have ever had, anywhere, anytime, ever. There are also some good places to eat on that same street, heading north.
We stayed once at the Nardizzi Americana, and it was okay, but the least favorite of the four hotels we have stayed at in Rome. Again, our main focus was food. We were on (yet another) RS tour, which stayed at the Nardizzi. And it was probably the least good (how's that for dancing around a word) breakfast I have ever had in a European hotel or hostel. As I recall, the coffee was terrible, from a machine (I have had good machine coffee; this wasn't.) And the first morning we were there the bread showed up as we were leaving. Few choices of much of anything. The next day my DH and I brought fruit and cheese to share with our tour mates, and the third morning other folks did, as well.
But I must say that that was a long time ago, and we did report our concerns to the RS office. It is quite possible that it's better now. And it's also possible that breakfast isn't as important to you as it is to me.