The only real difference is that Carnival, RCI, HAL, and the other non-NCL lines have assigned seatings for dinner in the main dining rooms. This is where you get either the main or late seating, with an assigned table, and you then need to show up for dinner at the same time every night and sit at the same table. People like this since you get to meet other people seated at your table, and have the same wait staff for the whole cruise. However, it has been our experience on some ships (the Holiday in particular) that we always get a table for 4 just for us (there are 3 of us). It is nice to have the extra space, although there is nobody new to talk to. I suppose if someone was traveling solo, they might get "stuck" with us, but it hasn't happened yet.
Princess has both dinner options. They have at least one dining room set up for assigned seating, and the other dining rooms are "personal choice" which is what they call their "freestyle".
But it is quite common on all cruise lines for dining room breakfast and lunch to be open seating, where you can come anytime during the open hours and be served.