Here is a post from a poster who asked the same question this week.
Here's a few questions:
Do want 7 nights of different menus or is 4 ok?
Do you want the same shows on both cruises or like 7 different performances?
They do play less movies in the theater, is that ok?
Do you mind not going to other ports?
More variety of games shows and activities on the 7 night, do you like those activities?
Reasons to do 3/4 day
If you are wanting to get to Gold or Platinum faster
Want the experience of B2B
Have time to relax and not worry about going to all the shows, but knowing you haven't missed anything
Don't care about destination
2 stops at CC
2 Pirate Nights
Reasons for 7 day
More menus
More variety of entertainment
More movies
No leaving the ship after 1st cruise
More ports
The cost should be relatively close, depending on the 7 day destinations and time of year. But most like, shouldn't be much of an issue. My recommendation, you can't lose! If it were me, I would do the 7 day IF at least one of the days is CC. I am just not a fan of all of the hustling everyone does on the 3/4 night cruises. A lot of people are just so bent on getting everything done and standing in every line. The 7 day just seems so much more relaxing after day 2 or 3. People really start enjoying themselves and relax a bit more. Just my opinion.
Total: 24 DCL Cruises / Total Nights on Board: 133 / Favorite Ship: Magic