We like to combine time at WDW and on the
Disney Cruise. We own DVC points, so the 3 night works out well for doing this. The points are lower on Sun-Thurs nights, so we can stay at WDW from Sun to Wed and then cruise Thur-Sun.
However, I have to confess, we do like the 4 night better than the 3 night. We like having a day at sea AFTER visiting
Castaway Cay. We can relax while we pack on the last day (it feels rushed to get cleaned up and packed on Castaway Cay day). I also like knowing that we have 2 chances to make it to Castaway Cay. If the weather is bad on Tuesday, they try again on Wednesday.
We can then go to WDW for 3 days after the cruise (but it takes more points to stay on Fri and Sat nights--if you are paying cash, then it doesn't matter which days you are at WDW).
One advantage I can think of for the 3 night is that you are less likely to experience sea sickness. The ship would only be moving at night and each day you are actually 'docked'. It is on the days at sea when the waves are high that you can get motion sickness.
But as to the question, is it worth it, I would say YES! You get the best of the best on the 3 night. You see all three of the Disney themed shows, eat in all three restaurants, and you visit Castaway Cay! Those are the BEST parts of the cruise! So just for those activities it would be worth it!
DJ