I just returned from a stay at the Fontanella Borghese, which I found to be an excellent choice. The hotel is in an ideal location - close to the Spanish steps as well as other monuments. It is also on a quiet street.
Our concierge was extremely helpful and friendly. We had considered renting an apartment, but I'm happy we chose the hotel because they made our trip much easier (making reservations for us, giving us information and recommendations).