I am not able to compare cruislines at this time since I've only been on Carnival so far, and have been very pleased with them. I'm going for my Platinum Status on Carnival and once I attain that I will probably try other cruise lines more often.
The Western Caribbean has been my favorite so far. I loved Belize and Isla Roataan and am going back again in March for my 45th Birthday on the Valor. I'm kinda leaning towards doing a Southern Caribbean next.