My very first cruise was on the Mariner's maiden voyage and talk about being spoiled right off the bat! Then our second cruise was on the Carnival Glory, when it was brand new too! Somebody please pinch me on the beauty of these ships.
They were both great, but different experiences. My husband does not prefer Carnival at all. I think it sort of depends on the mood you're in and what you are looking for in your vacation. Because Carnival is advertised a a "party ship", people tend to come in with that attitude and just let their hair down.
Wanting to book an "impressive" cruise for my grown kids, (2 that have never been on a cruise before), and still looking at a reasonable price, I decided to go with the Voyager of the Seas because I was so very impressed with the Mariner and they are quite similar. The Promenade is fantastic. We will be on this ship in 6 days and I have some Promenade rooms booked for the first time. I think this will add to the fun as well.
I also wanted them to feel more "elegant" on the formal nights since I am MAKING them all bring suits or tuxes and long gowns.
I just think RCI will be the right choice for a first time cruise impression.
Like I said, it just depends on where your mind is at when you go. From some of the crazy, (in a good way), people I met on Carnival, I'm sure they would much prefer Carnival over RCI.
Since my kids are into the nightclub scene and dancing, I thought maybe Carnival would be better from the party standpoint, but I still chose RCI in hopes that the nightclubs would be hopping and if not, they'd be the ones to get it going. Ha! (My oldest son, 27 is a DJ and spins in a lot of clubs).
Hope this helps,