Hi E. Although I haven't been onany of the ships listed above, I think I have enough knowledge to share my opinion...
The Norwegian Cruise Line's ships offer "Freestyle Dining"- dine anywhere anytime and offer many restaurants to chose from. If you are a fan on food, NCL will be a good choice. But since you listed three Norwegian ships, I would chose the DAWN. She is a big ship; and has a big teen lounge and kids facility. She is big with many open decks. I heard many teens go onboard her. The SEA just got one with a refublishment, so she will be clean and have new furnitures. The Majesty does not have much uniqueness, but bigger than the SEA.
The Carnival Pride is from Carnival Cruise Line and they promote "Fun Ships". I have been on one of their ships and I had Lots of Fun. The PRIDE is very colorful inside. The teen facility is not as big as the DAWN's, but many teens onboard.
The NORDIC EMPRESS. Oi. She is a small ship and does not offer much facilities for teens. She has a small kids room, rather than teens room. Considering that she was built in 1988, many interiors is quiet "out of date". If you want to try a small ship, I think she will be better than SEA.
Overall, I would pick NCL's NORWEGIAN DAWN. If you have any questions, feel free to AIM me, daswimma100. Or email me at firstname.lastname@example.org