Some of it will depend on what you like most about the Dream/Fantasy that might not be on the Magic. Some will depend on what you have come to expect. Our first cruise was on the Wonder and the next few on the Magic. I was worried about being disappointed with the Dream over the Magic. We've done all four and I like all four but I still prefer the Magic over all others. I don't like the larger ships as much although there are things about the Dream and Fantasy that I prefer to the Magic but things that I prefer on the Magic over the Dream/Fantasy.
I love Remy on the Dream and Fantasy but prefer Palo on the Magic to the ones on the Dream/Fantasy. I like the Meridian lounge and the champagne lounges on the Dream/Fantasy but I like the piano bar on the Magic better to the piano bar on the D/F. I like the adults-only pool on the D/F but the prefer the adults-only hot-tub on the Magic. I prefer the Cove Cafe on the Magic. I prefer Animator's Palate on the Magic to the ones on the D/F. I liked Satellite Falls on the Fantasy.
I prefer the staterooms on the Magic in both size and layout. I like the atrium on the D/F better, including the lounge in the main lobby which I think is a nice touch. I like the midship detective game on the D/F. I like the layout of the theatre on the D/F better. I have no difference in opinion between the shows on any of the ships - I think they're all well done. I like the shops on the Magic better.
I like the cozier feel of the Magic and the easier debarkation as well as the smaller number of people at
Castaway Cay. I like the Aquaduck on the D/F (it was too cold to try the Aquadunk on the Magic on our last cruise which was in the Baltic). I like the greater option of ports on the Magic, not just because of their specific itineraries, but because they are able to get into more ports without using tenders.
I think the elevator lobbies are better placed on the D/F than the Magic (although I hardly ever take the elevator on the Magic because there aren't as many decks to worry about so I take the stairs). I prefer the gym and spa on the Magic. We liked the mini-golf on the Fantasy but I could take or leave that. It was there, we played it, we had fun, I wouldn't miss it if it wasn't there.
I actually prefer not to have the on-demand TV service that they have on the D/F because I find that they have a better selection of first run movies on the Magic (that may be just coincidental that I preferred the selections when we sailed but it's a valid point for now until proven otherwise). I preferred the food selection in the MDRs on the Magic. I used to prefer the buffet restaurant on the Magic until they messed it up and copied Cabanas from the D/F so that's no longer an issue.
I think I prefer the Magic overall partly because that's the class that we sailed first and for most of our cruises. You may find that your preference will always be for the Dream class because that's what you're used to. I know that a large part of our preference for the Magic is due to size as I really don't like the megaships as much and the rooms are a bit more spacious than the Dream class. But I'm happy to sail any of them - as you can see, there are lots of things to like about all of them.