We sailed on the Magic and loved it, but we loved the Fantasy even more.
Pros of the Dream-class: The Dream-class ships have features the Magic doesn't, such as mini-golf, virtual sports, Aqua Duck, Midship Detective Agency, Remy, Rainforest spa with a view out to sea, on-demand Disney movies in the staterooms, virtual portholes in inside staterooms, etc. If you'd really miss any of those, I'd sail on the Dream. If you're gluten-free, I recommend sailing on a Dream-class ship. I also remember a slight sewagy odor in our Magic cabin bathroom that we never had on the Fantasy. But it was nothing terrible and the scent didn't go into the rest of our cabin or anything. We liked the port at Port Canaveral (Dream) better than the one at the Port of Miami (Magic), and I like the Orlando airport more than the one at Fort Lauderdale. If you're sailing the Dream, it's easy to tack on a few days at WDW.
Pros of the Magic: We liked Animator's Palate better on the Magic than on the Fantasy (though I loved the latter's animation show). My son preferred the Magic's Oceaneer Club & Lab to the one on the Fantasy (the latter was more crowded). We also much preferred the 'Til We Meet Again event on the Magic to the one on the Fantasy. If you're sailing the Magic out of Miami, you might have a stop at Key West, which is reportedly very nice. They're both fine ships. Choose the one that most appeals to you and you'll probably love it.