I sailed the Wonder back in 2000 then sailed the Magic 9 times after that. I LOVE the Magic. We were on the Fantasy last month and she was magnificent. Before we sailed we were up in the air about what to book for next year. We looked at two 5 night back to backs on the Wonder, but decided to reserve judgment until we experienced the Fantasy. It took no time at all to decide we want to spend more time on the Fantasy and really get to know her.
That being said, all four ships are awesome and have their own pros and cons. I would not hesitate to sail on any one of them. However, my biggest criteria is the length of the cruise. I don't really want to do less than 7 nights, more would be better, so that's usually my first choice. I don't think you can go wrong with sailing any of them.