Basing yourself right next to the Vatican makes sense if that's your priority, but it adds a longer trip to the Colosseum each way. The historic centre and Monti split the difference, keeping both sites a reasonable, similar distance away.

Monti

A characterful neighbourhood immediately behind the Colosseum, popular for its restaurants and easy metro access toward the Vatican.

Historic centre

Central to everything — walkable to the Colosseum, a metro ride or 20–25 minute walk to the Vatican.

The metro connection between the two sides of the city runs through Termini, adding a change most visitors don't expect from looking at a map — budget a genuine 25–30 minutes door to door between a historic-centre or Monti base and the Vatican, not just the walking time to the nearest station.