I have sailed a lot on Carnival, never on Royal. Not that I have anything against them, but they were never convenient, or in my budget. I don’t like the larger/largest ships, so don’t think I will sail Mardi Gras, and that pretty much rules out the Oasis class. My question is, why have you chosen a cruise? Are you going for the ports, or for the floating resort? I don’t think a first cruise should ever be on the biggest and newest. There a lots of ships with plenty of amenities that are a bit smaller, and newer.
Also consider the ease/cost of travel to the departure port. And the cost of the hotel the night before, because you should always fly in a day early, especially in winter. EM