Perhaps it will come as a surprise to some, but I think the cabins are more comfortable on the Carnival Liberty. We had a Category NS (Century Suite) on the Century, which though not a true suite, does comes with butler service. On the Carnival Liberty we have a standard balcony on Deck 7 (Category 8A I believe). The cabin on Liberty is larger, brighter, and laid out very well, and perhaps also surprising, I think the bedding on the Liberty is superior to that on the Century, with more comfortable beds as well.
Though we certainly miss the size of the balcony, and the location, on the stern, from the Century.
Comes as no surprise to me. We recently sailed on Carnival Glory (our 4th Carnival cruise) and have sailed on various Celebrity ships 7 (or 8 ) times. I like the rooms on Carnival better. They appear
larger . . . and whether they are or not, IMO they are more comfortable. Brighter and cheerier. The bedding (on Glory at least) surpassed Celebrity's hands down.
I am not a huge fan of the Carnival cruising experience, but do feel that in this category, they have got it right for the "mass market". All in all though, I rate our experiences on Celebrity quite a bit superior to Carnival.