We have cruised twice before on Disney once in a cat. 6 and once in a cat. 5. Three weeks from Saturday we are going again in a Cat 6. There is no difference between a five and six except for deck number. Personally I did not notice ANY difference between decks six and seven. A Cat. 4 is basically the same except for a few extra feet at the end of the cabin to make room for a murphy bed. (which you would not need as there are only three of you)
Personally I would not spend the extra for a few feet, I wouldn't turn down an upgrade but I would not spend 400.00 for it- heck that almost covers our bar bill!