Our first 3 cruises were with Princess. I agree with trip about Princess being more elegant. And though it has been awhile since being on Princess I loved their food and find Carnival just as good but not as fancy.
I did find that we liked the entertainment more on Princess but Carnival's is not bad.
Star Princess 2005, Sun Princess 2005
Caribbean Princess 2006, MSC Lirica 2006 , NCL Pearl 2007, Majesty of the Seas 2008, Carnival Destiny 2008, MSC Lirica 2009, Carnival Valor 2009,Carnival Legend 2010, Carnival Liberty 2010,Carnival Fantsay 2011,Carnival Valor Feb 2012, MSC Poesia Dec 2012, MSC Poesia April 2013, Carnival Legend Nov 2013, MSC DIVINA Jan 2014