Paradise is a newer ship by a few years, one of many Fantasy-class ships built by Carnival. However age is not always a factor since all ships get freshened up with new furnishings, carpet, etc., every so often.
Using the items you listed, the only thing I can say is the Monarch standard cabins are 119 square feet, while the standard cabins on Paradise are 185 square feet. But bear in mind for a 3-night cruise the cabin size isn't a major factor. You also are rarely in the cabin. As they say, when the lights go out and you close your eyes, all cabins are the same size.
To be honest, the itineraries for both ships on the 3-night cruises isn't memorable, unless Ensenada excites you. Both itineraries are the same.
Monarch is being moved to Florida in the fall to replace the Sovereign in the popular Bahamas cruises from Port Canaveral. Personally I'd do the Monarch before she leaves California.