I've been on Voyager, Explorer, and Adventure. They are pretty much all the same, ship wise. I would pick a Voyager class ship, and choose the ship based on what you want to do. Look up the shore trips for each island (use RCCL's site, and I like www.shoretrips.com
as well) and book based on that.
If you like to party....go to an Island that has a well known party spot, Like Carlos 'n' Charlies. (I know aruba has one, as well as Cozumel)
Just don't plan on doing anything early in the AM the day after that port.
I hope this helps. Post some additional information about your hobbies, interests, I think we can help you better. (eg: do you like Scuba, Snorkle, or do you like to stay on dry land, or is the drinking, party thing more your style. etc)