Being the first cruise, either one is fine. You have lots of people who like or dislike for personal preference and no mean one is better than another. Pick one this time and take the other one next time. Comes down to which port you like to go first?
We cruised on Carnival (2X), RC (1X), NC (2X+1 next month). Here are my opinion:
Carnival - Least expensive and attracts younger crowd and young families. Lots of partying on pool and drinking on deck (some may say more lively) . Teenagers seem to like it more. It was the first of our 2 cruises and remembered the food was good, but people say the quality has gone down.
RCI - Is the least favorite of the 3. Food in the dinning room was very good ~20% of the time. Food in the buffet was ok the first night or two and then everything seem to taste the same. Mostly taste less chicken with different source on top. Food overcooked and dry. The serving area is small and very crowed.
NCL - Dinning room food was good in general, may be very good 15% of the time. The specialty restaurant were better, at least the first cruise. The first cruise, they offered 2 for 1 special. We try them all, except for the steakhouse. We may try the steakhouse next time. Probably do French or Italian again and can pass the rest. The buffet has more selection and the dinning area is more spacious (larger ship). NC is less formal and can walking any time during dinning hours. We like it, but some dislike if for the same reason. The evening show was comparable to RC and better than Carnival. The chocoholic event was amazing and not to be missed if you like sweets. Long-long table of chocolate deserts. First cruise was in the afternoon, the second was in the evening. We also find the crew more friendly.
Need to try the Princess (2014).
Happy cruising!