To each his own - while the Quantum Class ships (5,600 passengers) are “amazing”, they are mob scenes. Oosterdam is well laid out and her 2,200 passengers have a lot more room. I have sailed both lines enough to have a strong preference for the food and service on HAL.
However - Rome is a powerful draw - if this is likely to be your only Med experience, you should look at itinerary. But do not think a day or so is enough — if Rome is you deciding factor, give it at least two or three days.