Why not choose because of the ports that are visited? Do you or your friend have any preferances of where to go? Choose the itinerary that you like and then narrow the list down from there. There are lots of things to consider when choosing....ports, # of days , price, maybe where it leaves from (airfare if needed), if you want a different port each day or if you want some sea days to relax, etc. I have only cruised Carnival and I love it. We are taking a Princess cruise next year because Carnival does not travel to Aruba. I am 51 and Carnival has everything for a younger crowd plus enough fo everyone else to do too. Their Camp Carnival is supposed to be the best if you are travelling with children. They are the only line that takes them at 2 yrs old.
I think finding the right one is the hardest part but it is fun to see all of your choices and then compare them! Have fun!
Glory - 2006
Liberty - 2007
Pride - 9/28/08 - 30th anniversary cruise
Miracle - 12/7/08