We are going on our second "just cruise" in August; a 7 day. I guess we don't do a 7 day w/ a WDW visit b/c it's hard to take off more than one week of work at a time (although this time we will take a day at a Disney water park first).
We do the 7 day b/c I LOVE the ship and want to be immersed in the
DCL culture when I'm there. I even prefer the Eastern just to have that extra day at sea (one less port!).
Our first DCL cruise came a year after we had been to the parks, so the kids "had just been there". You don't mention the ages of your children - if they are elementary and had never been to WDW, I would suggest going to park only to immerse yourselves in WDW magic! If you've been to the parks, and they really want to do parks again, then a Land/Sea is a great sampler!
BTW, I still go back to WDW every fall for an "adults only" trip - I tend to forget the kids haven't been there in about 2 years!
We have family near Orlando, so I know they'll have another opportunity to go to the parks in the future.
So, I've rambled (sorry!);in short:
Everyone should have the opportunity to do the parks at least once!
Land/Sea is a great opportunity to do both.
For me, a 3 or 4 day would be too rushed (again, that's just for me!); I love having the 7 day cruise to just "live" on the ship, experience that other aspect to Disney. Just the difference too, I guess, of whether you want a theme park vacation this year or a cruise.
Either way, you won't go wrong!!! Happy Planning!