We too have cruised Carnival while in our twenties( twice) and then switched to Princess.
We like the food better on Princess, we like the pools and pool areas better on Princess( the oungers are padded not just netting) ,and one of the most important differences( to us) is the staff attitudes. On Carnival we found that many of the staff we encountered day to day( not the cabin steward ) just had a sour look on their faces, like they didn't like their job, and resented the passengers. We don't recall any of them ever smiling at us and just saying hi if we passed in a public area. Sure the bartenders could be your best friend, but it always felt like unless they benefited directly from your patronage then they just weren't interested in you.
We saw cleaners who, when we would say good morning to them , they would look at us like we had two heads.,most of the staff just seemed sullen.
On Princess we are almost always greeted( a smile ,a hello) , and it is the little things that count.
Would I cruise Carnival again, maybe, they do have large cabins, and I have heard that the food has improved( although we LOVED Princesses Anytime dining program) .
I would choose a new ship, and I would avoid school breaks, and I would NEVER take one of their 3 or 4 day cruises.