I have only cruised Carnival and I have nothing but good things to say about it.
My husband has also cruised Norwegian and had a crappy time. The A/C went out and everyone slept on deck!
We are looking to cruise again in the not-so-distant future. I think we will look based on cruise route and price. We live close enough to NYC to cruise last minute, which is a great way to save mucho dinero!
Also, which line you pick should depend upon your tolerance for certain kinds of atmosphere. For instance, we don't mind seeing kids running around and we love to be casual so Carnival was okay for us (seemed very campy
). But if you are looking for something formal, I would suggest one of the higher-end lines.