I know how you feel. I have been on Carnival once and my husband has only been on NCL. We have had such great trips on NCL that its hard to try another line. I have found that NCL usually has a better rate and so far I've liked their itinerarys better.
I think its more important to find the intierary that suits your needs at a price you can afford rather than worrying about which line you are on. Every line has good and bad things about them.
For us we will probably stick with NCL unless another cruiseline has a better itinerary and price.
Carnival Inspiration, Sept 2009
NCL Sky, June 2009
NCL Sky, Nov 2008
Disney Wonder, April 2008
NCL Spirit, Sept 2006
NCL Pride of Aloha, March 2006
NCL Dawn, June 2004
Carnival Sensation, July 1998