There isn't much difference between the two ships (or any of the Carnival ships for that matter) in the way of nightlife. Focus on the itinerary of the cruises and pick the one that you would like to try the most. The ship will take care of itself.
As far as the length of the cruise goes, there are many opinions on that. I personally think that you need a minimum of a 5 day cruise to call it a true vacation.
Oh, and WELCOME to cruise chat!
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