I'm in the minority here but I say do the seven day.
We did our first cruise last Dec, a seven day, and loved it. My husband was fine but I was a bit queasy the first day, but never got sick. Our seas were pretty rough with a late season tropical storm off to sea near us. But after that first night and some distance, I was fine.
This year, we did a 4 day on Dec 1 (in lieu of the cancelled 7 day eastern) then followed with a second previously planned 7 day western cruise. We're not rich, just members of DVC. Anyway, we felt that everything was so much more rushed on the 4 day that it was hardly worth it. Maybe we got spoiled on the Magic, but the Wonder was louder with many many many more bratty kids. The pace was just frantic to offer everything to the guests. You never really had time to take advantage of what was offered to the fullest. The 7 day was just much more relaxing in our opinion.
Just my .02 worth.
Lisa Marie Smith