7 nights on the Magic, hands down. We love the Magic and would take her in a heartbeat! One nice thing about the Magic is that she was given a number of upgrades a couple of years ago to bring it more in line with the newer ships. StudioSea was replaced with the D Lounge, Cabanas was put in vs the old cafeteria style buffet, the slides got major upgrades (if you like the traditional slide, the "Twist-n-Spout" slide on the Magic is the best of the 4 ships, IMHO. Also, the Aquadunk is fun as well.
However, one BIG thing I would say is that if both visit Castaway Cay, your experience will be much more relaxed there on the Magic vs. the Dream as you are sharing it with 1,400 less people (not that it's bad on the Dream or Fantasy as the island was expanded to handle the crowd level, it's just that you seem to have much more room to move around on the classic ships while there).