I wouldn't hesitate to sail on any of the ships.
Yes, the Dream and Fantasy are larger - the Magic and Wonder seem intimate by comparison. It's a somewhat longer walk from place to place. It's harder to feel a sense of community - more guests, more staff, so more faces seem unfamiliar throughout the cruise.
However, they've also been able to make public spaces substantially larger (larger than the 50% increase in passenger capacity), so you're less likely to feel crowded anywhere on the ship, such as in the lobby prior to dinner, during deck parties... and Cabanas, the pool deck buffet is also very spacious - no more traffic jams or frustrating hunts for tables. And then there's the all-new stuff like the Enchanted Artwork/Mickey's Midship Detective Agency, the new Animator's Palate shows, Remy... And if you did have kids along, the improvements and increased space in the kids/tweens/teens areas are dramatic. The spa has been upgraded, and the adult entertainment district is greatly improved (more choices and great theming). Altogether, for me the pluses far outweigh the (for me) small minuses.
Co-Author, PassPorter's Walt Disney World, PassPorter's Disney Cruise Line, and PassPorter's Disneyland and Southern California Attractions