Rumor is that the Fantasy will take over the 7 night cruises, but the only thing we know for sure is that both ships will be based in Port Canaveral initially (per contract with port.) MANY rumors and hints about what Magic will do, but no official announcements yet.
Most of us who are
DCL freaks do prefer longer cruises, but the line gets more "excess" revenue on the shorter cruises--more money in excursions, photos, spa, etc. over and above the cruise fare. The frequent cruisers among us might do one spa treatment on a 7 night cruise, while people on the shorter cruises may try to do one on their cruise, giving 2 for the week. Unfortunately for DCL, those of us who have sailed many times spend less in the gift shops, Shutters, etc. as we know what we want and where we find the best value. We're also more savvy about booking excursions on our own, etc.
Shorter cruises = more passengers = higher per day revenues.
In addition, many people will sail the Dream just because it is the new ship, not caring so much about what it is doing as just wanting to be on board the "new" one.