I’ve stayed in that area before (at Albergo del Senato). It is very central to the main sights, but also quite loud from fairly early in the morning to fairly late at night. Just FYI in case that bothers you.
It is a pleasant 20-ish min walk from the Excelsior to the Pantheon area, and the St Regis isn’t that much different. Both are in much quieter areas.