Originally I would have said the 3 night cruise would be the cheapest. However, when my extended family was looking to book a cruise for this spring break, we were originally looking for a 3 night out of Port Canaveral. I stumbled upon the 5 night out of Miami for the same week, and it was almost the same cost as the 3 night. I guess it's because the 3 night was on one of the newer ships and the 5 night is on the Wonder. Either way, we were thrilled to sail for 5 nights!
My husband, kids, and I have sailed 7 night cruises several times, but the rest of our crew has only done the 3 night, so they are excited beyond belief to get to do the 5 night.
I guess my point is to check all itineraries. We were pretty certain we would be doing another 3 night and were so happy we found the cruise we ended up booking!
ETA: I went back and looked at my notes and the opening prices for the cruises we were looking at. So you know, we are booked on a 5 night on the Wonder in a 5C. Our cost was $3509 for 2 adults and 2 children (17 and 14). If we were to have done the 4 night on the Fantasy, it would have been $3902. The 3 night on the Dream was a little over $3000. So, we would have saved $500 by doing the 3 night, but we would have spent an additional $400 if we did the 4 night. Just wanted to let you know. Check the older ships and their itineraries. They are often cheaper, and we love them.
Susan