Just got back myself. You have to try Pizzarium, it is somewhat near the Vatican on the corner of Via della Meloria, and Via Cipro (on Via della Meloria). If you don't have a car, there is a metro stop right there.
It is far and away the best pizza a taglio I have ever had. If I get some time I will post some picture we took when we were there.
As far as pizza napoletana, we had just spent close to a week in Napoli, so I did not see any reason to try pizza napoletana in Rome, but pizza a taglio in Rome is definitely worth trying.
Let me know what you think.