We did our first cruise in Feb 2001 on the 4 day Wonder with 2 adults and DD8 and DN11. I posted this exact question and got relatively the same responses. In the end we chose the 4 day because DH was a little aprehensive, his only cruising experience being the Navy;~)
Our cruise had a Freeport stop so we stopped somewhere every day. The first stop was Nassau and we went to Atlantis. Fun, but tons of rushing to get there, lots of walking, and "hard play" all day. Then Freeport, got off and did some shopping and got both the girls hairbraided which took most of the afternoon. Then CC, wonderful, warm, great day.
I was so completely exhausted and FULL after these 4 days. I never once laid on a lawn (woops, make that deck) chair, saw a movie, mosied around. It was like, get up, get off, do what we have planned, get back, get to the show, have dinner, etc. etc. etc. I was DEAD! When people say to do land, sea with sea last so you can relax, I alway laugh to myself. In my wildest dreams I cannot imagine doing a cruise AND WDW in 7 days and still be able to stand to board your flight home!!
On our last night, I polled the family to see if anyone was sorry we hadn't done the 7 day and all votes were happy we'd done the 4. I strongly feel this was because we were heading to a very large condo the next day and a 7 day vacation in Orlando with lots of fun things planned like Univeral and Discovery Cove which we were able to take at a much more relaxed pace. We were very ready to rest, sleep, and spread out.
Having said all this, I realize our experience was due largely in what we did with our time. If we had taken it slower or had a day at sea, we might not have felt the same way. We have another 4 day booked for Feb and I have planned NOTHING. We are just going to go with the flow. I intend to lay on some comfy deck chairs, stroll around the ship and sleep in if I feel like it!! We may have considered the 7 day but by the time we booked it was well over twice the price of the 4 day.
So much depends on the way you travel, the ages of your kids, if you are adding on any time before or after your cruise. I think it would be a huge letdown to jump on a plane, cruise for 4 days and then head straight home.
Good luck with your choice... and when you do cruise, don't try to do everything as I did. It'll kill ya :~)
Jenn