Haven't been on Princess since 2000, but did Maasdam and Mercury in the same month.
The Millie class ships will compare quite favorably to Dawn Princess. We had a baclony stateroom on the Summit in 2002, it was very nicely laid out. We could have done several weeks in it. Had a balcony on Mercury, and because it was fairly forward, where the ship narrows down, it was only 158 sq ft, not the 172 sq ft as advertised. However, it was nicely laid out.
The difference between HAL and X, as we see it, is the style of decor. HAL is quite dark, richly old-world European. X is brighter and more contemporary.
We did not find the booze-guzzling college frat boy group on X. I am assuming you are referring to what Carnival was famous for ten years ago????
Don't know how old you are, but our group ranged from 54 to 90, and we all were happy on the Mercury, and thrilled to pieces with Summit. The Maasdam cruise was about a month before drydock, and she really needed some TLC, like replacement of light bulbs in the dining room lanterns, worn carpeting.
But we would go on any of the three again.