Being that it's your first cruise I say go with the itinerary that is most appealing. Just remember cruising is not for everyone, but everyone should give cruising at least 1 try. You just don't know until you've gone what a cruise does for you, it could either be your best vacation ever because of the choices offered or it could be the worst because of how you body reacts to being on board. So go with what you do know, which is what ports interest you. With an appealing goal in mind sometimes that notion can help the the brain over ride the body.
As for features of the newer ship's more being expansive since you've never experienced them before you won't real realize what your missing. It's a little like those that have always traveled in an Ocean View or Interior cabin experiencing a Balcony or Suite for the first time, and for most once you've done that the lessor cabin option is no longer even a thought. So save the lessor desired itinerary on what you think is currently the better ship for a later cruise once you've discovered what cruising does for you.
Vacation Countdown Clock Generator
April 2013 Carnival Splendor
September 2012 RCI Explorer of the Seas
May 2012 Carnival Miracle
July 2011 Carnival Glory
October 2010 Carnival Pride
September 2010 Carnival Glory
Aug 2003 RCI Sovereign of the Seas
July 1992 Premier Oceanic
June 1992 Premier Oceanic
May 1990 Premier Atlantic