Some people think that because they are higher in the ship they have more status. There really is not difference on today's cruise ships. Used to be that the lower you were the smaller the cabin. (But may still be true to a certain extent on some older ships.) But like you said, unless you are going to a Balcony cabin, there really is no difference between cat 6 cabins. Inside cabins are about the same as long as the cat Number is the same. Thats not to say that there isn't different sizes amongst cabin types. But for a given type, the deck of the ship won't matter.
As far as ship motion goes... it has to be pretty rough for you to feel more motion on an upper deck versus a lower one, but some people may be more sensitive than I am....
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013