The floor plan inside is identical for cat.5-7.
Cat. 5-6 are also identical as to the veranda the only difference is which deck they are on, cat.5 is only on deck seven but Cat.6 can be on deck 5 or deck 6.
We have stayed in a cat.6 twice and a cat 5 and there was no difference at all in the rooms. Infact one Cat6 was on deck five while on a different cruise we were on deck 6. I did not notice any difference in "motion" what have you from deck 7 vs. 6 or 5.
A cat. 7 cabin is a navigators veranda. What that means is it has an obstructed view. This can either be due to one of the four reclassified cat. 5/6 or the "port hole" verandas, all are aft.
By porthole I mean the traditional navigators veranda, instead of being wide open with a plexiglass or metal rail they have a wooden frame and a large open porthole on the veranda.
some people prefer them as they are more private.