The 3 night does go fast, but you get the BEST of the BEST on the cruise. You see all three of the Disney themed shows, you dine at all three restaurants, and you visit
Castaway Cay. Those are the main highlights of the cruise.
The 4 night is nicer because you get to relax on the last day with a day at sea. We really like that better than visiting CC on our last day. We don't have to rush back and get packed up that night, we can take our time the next day. So I think there is a BIG diffrence between the 3 and 4 night cruise.
That being said, we are Disney Vacation Club members, and we like to visit WDW during the same week as the cruise. Because of the way the DVC points work, the 3 night cruise actually works better for us. We can stay at a DVC villa from Sun-Wed nights and then go on the 3 night cruise from Thur-Sat nights and fly home on Sunday.
We are doing a 4 night next March, but we don't have any points left for our use year anyway, so we will pay cash for our WDW rooms this time.
All the cruises are nice, and no matter which cruise you pick, you will enjoy it.
DJ