Most of my cruises are with Carnival, however, I have done cruises with Princess and RCCL in the past year. I have also sailed with NCL and several other lines that are now defunct.
That being said, for the availability and value (gotta love those military discounts), we will stick with Carnival. Besides, at this point we are Platinum with them, and the extra perks make it even more enticing.
BTW, one of my RCCL cruises was Freedom of the Seas and that was too big for me, so I doubt you'll ever see me on the Oasis class ships.
I will try the Carnival Dream when it comes out, but I hope Carnival stops at that size. I would hate to see all cruise ships turn into floating mega-resorts.