We have sailed Carnival many times and we always try and get a cabin on deck 6, 7 or 8 at the very back of the ship. The last 1 or 2 cabins are L shaped when looking at the diagram. These cabins are generally a bit larger and the balcony is almost 2 times the size of all the other balcanies, for the same price.
Also Empress deck is always a great deck just because there is a floor of cabins above and below to buffer the noise. we always get a balcony but if you are getting an inside cabin look to deck 7 or empress. This is generally where the suites are located and the best cabin stewards are always given this floor. therefore you will get better/friendly service from your cabin steward.