I would highly suggest Co-Roma for your stay in Rome.
This apartment was perfectly located to all the sites. It was within walking distance or a short bus/tram ride away. Constanza was a perfect host.
The only negative, which cannot be controlled, was the night noise that came from the bar or walking traffic, but really the location cancels out this minor issue. If you are a light sleeper, bring ear plugs or use ambiotic noise app on your cellphone.
The bed was very comfortable and had great pillows.