We did the 3 day Disney Wonder in Nov., 07 and the 4 day Disney Wonder in Nov., 08. If it fits your budget, we think that the 4 day is clearly the way to go. The extra day is a sea day, usually after the Castaway Cay day. It allows you to relax after spending the day at the beach, instead of rushing to get cleaned up for dinner and pack the bags, which need to be out for pick up the night before you disembark. Also, on the sea day Palo offers brunch (NOT to be missed!!) and there are loads of activities, some (like a galley tour) not offered on the 3 day trips. Let me know if you have any other DCL questions.
DCL charges a premium compared to some other lines...but we think it's worth it! All cruises, and cruise companies, are not equal. Of course, it depends on what you want out of your vacation.