I thought I would weigh in on this.
We have sailed in both a Cat 5 and Cat 6. I agree completely with outahere, in that there is no difference between those two categories. I would have never known that we were on a different deck when we dropped to a Cat 6.
DH and I thought breifly about a Cat 4. However, with just DH, DS6 and myself, Cat 6 has plenty of room. If we had another child, I might feel like I could use the room, but not in this case. DH and I spend a lot of time out on the verandah, which makes the cabin seem even rommier (sp?).
So, depending on the size of your family, I vote to save the $$ and spend the money on bahama mamas or konk koolers. LOL