We are also Diamond on RCCL and Platinum on Carnival. RCCL is a much better deal as far as the drinks go since we get four free per person per day. We do not get the drink package because it would not be worth it for us. That being said 15 drinks on Carnival would equal about seven on RCCL. Just got off the Magic a few days ago and they make terrible drinks, lucky to get one full shot in a drink. If you find a good waiter, maybe in the Comedy club, that makes the drinks themselves you can get a good one with an extra tip.
The fast food type restaurants that are included with Carnival are a big plus and vary from ship to ship. Sounds like you will be on a Mardi Gras class and Shaqs chicken, Blue Iguana and Guys are all great as is the Pig and Anchor. I believe there was also a great Asian restaurant on that ship that was delicious. Our favorite is the Mongolian Wok but it is not on that class of ship.
Trouble with a lot of these restaurants is that they are closed at 6 p.m. or before and only available at lunch, Blue Iguana does breakfast too.
I do think that CCLs lunch buffet and all of the free eateries put it ahead of RCCL for lunch choices but the dinner option is not as good as lunch choices.
The cruise directors are a big plus on CCL.
All just opinions of course. RCCL and NCL both end up being a cheaper cruise for us because of all the good tier benefits.
We end up spending around 5 -6 hundred dollars onboard CCL that we do not have to spend on the other lines because of the included tier perks.