You'll likely find Carnival and Royal Caribbean more alike than different. I personally feel Carnival has better buffet choices, but that both lines are equal in the dining room. Carnival also has larger standard cabins. I don't know what category cabin you had on the Majesty, but if it was a standard inside or outside it was 119 square feet. On the Sensation the equivalent cabins are 185 square feet.
I encourage you to try other lines. It is the only real way to find what is most comfortable for you and your expectations.
Personally I like both Royal Caribbean and Carnival about equally.