I have moved this thread to the General Cruise Chat forum since I think it fits better there.
Usually, it's "the longer the cruise, the older the passenger mix". So, for that reason already, an eight-night cruise will usually have a younger passenger mix compared to a 14-night cruise.
As far as cruise lines are concerned, I would recommend either Carnival or - even though it is not among the ships you list - one of Royal Caribbean International's "Voyager"-class ships, or even the company's new "Freedom of the Seas" arriving in the Caribbean in June.
I've been on the "Carnival Liberty" and I did find the rooms to be very good. Actually, the standard cabins onboard "Carnival Liberty" are a bit larger than those onboard "Constellation" (which, however, has significantly larger suites) and considerably larger than those onboard "Sea Princess". Also, I've found Carnival's food to be very good for a standard-market cruise line. Celebrity in particular is renowned for its good cuisine, but it not like "Carnival bad - Celebrity good" but rather "Carnival good - Celebrity even better".