Ditto on the Aberdeen. We've stayed there a few times, and love it. The last time we were there, we took a bottle of wine down to the desk in which we had broken the cork; they not only extracted the cork, but right after we got back to the room the desk clerk showed up with two beautiful wine glasses!
The breakfasts are good, as are the computer and wifi. Since we prefer not to carry electronics with us, we appreciate a guest computer.
And like Holly, we walked all over Rome from there. The first time we went to the Vatican we didn't walk, since we were with an RS tour group, but we did walk back to the hotel from there. I don't know that I would recommend it, but it is doable!
And we too have had no experience with the Oceania. We have stayed at two other hotels in Rome in that part of town, and Aberdeen wins, hands down.