Great stay!
Testaccio is a really nice neighborhood with a lot of food options, and excellent transit access from a few blocks from the flat. I would highly recommend staying at Maria Cristina's.
The place was well furnished, had everything we needed (although if you're picky I'd bring my own shampoo) and had AC which is much needed in Rome in the summer time. And Maria Cristina is really quick to respond to your questions.
PS. Be sure to have breakfast at Linari, a snack at Trapizzino, lunch at Lo Scopettaro, gelato at Panna & Co, aperitivo at Volpetti and cacio e pepe for dinner at Felice!
Cheers