There are differences between Carnival and Royal Caribbean, but I wouldn't call them major. I am quite fond of both cruise lines and the differences between them have narrowed over the past few years. Carnival offers a great cruise product in my opinion. Royal Caribbean probably is a little better in the dining room, but then again Carnival is ready to roll out a new program featuring menus developed by a famous chef from Europe. The same chef has trained Carnival's culinary people on how to prepare and present these new items. I am not sure if it will be rolled out by May. However, the current Carnival dining room fare is quite good. I think Carnival blows Royal Caribbean away in the buffet area. Best pizza at sea - 24 hours a day.
Royal Caribbean has a better reputation for the entertainment, and I find the bartenders on Royal Caribbean to be less rigid. Carnival keeps a tight rein on their bar staffs - no variations from the recipe permitted!
Cabins on the Sensation are good-sized....at least 185 square feet which is the Carnival standard. Royal Caribbean has smaller cabins in general, although the Royal Caribbean cabins are more attractive. Carnival tends to stick with the basics.
I think you will be happy with your cruise on the Sensation. In fact, I am planning on cruising on the Sensation after it returns to New Orleans next fall.