Before our first cruise in 2013, we were also deciding between two different cruise lines, one with a large, new ship and one (NCL) on an older, smaller ship. Our travel agent gave us great advice: "On your first cruise you will be so in awe of the luxury and wonder of the ship, you won't know the difference whether you're on the biggest, newest, fanciest one or not. Save your money, go on the smaller ship, and save some wonder and awe for your next cruise." She was absolutely right. We had no idea what we were missing anyway, and we thoroughly enjoyed our cruise. We've been on bigger, newer and fancier ships since then, but we didn't necessarily love them more. There were good and bad about both of them
Also, although almost all of us base our cruises on itineraries, don't base your decision solely on that. Ports change, ports get closed, weather prevents the ship from reaching port - lots of things can happen that change your itinerary and stop you from getting to that one special place you planned your whole trip for. Just keep that in mind.