Definitely chose the 6 night. Your stateroom category is not that big of a deal in the grand scheme of things. Your cruise length is.
You won't notice much of a difference if you compared the staterooms, but you'd feel a huge difference if you compared the 6 night and the 4 night itinerary.
Also, 6 nights is a good length. Everyone is different but to me, 5-7 nights is the sweet spot.