My dad and I have been going for 14 days at a time since '05, twice a year (April/May and Oct/Nov/Dec). Before that, we'd only go for a week in early March.
The difference between trip styles is night and day. With one week you are constantly on the go. If you get sick or tired or something unexpected happens it can turn a trip into a disaster. When you're living there for a while then those inconveniences are quickly forgotten.
One thing you will notice is that Christmas week the parks will be exceptionally full and attraction wait times dramatically increased. We did fine park touring at New Year's but that was because we were able to do the late night EMHs (up till 4-5am). I doubt your kids would be up to that schedule. It's for nightowls.
Depending when you go, you may find you ride more the first week and just enjoy the ambience more Christmas week.
Plan on stocking your kitchen with lots of snacks and bring extra laundry supplies. Sharing the Christmas tree is another good idea to make it feel like home. We bring some lights to string around the room and balcony. We also bring a library of entertainment stuff for chilling in the room (e.g. books, DVDs, etc.). And of course I always have my travel laptop so we can check in via internet regularly. (If you have a lot of electronics, a small travel surge protector is very handy. We use a little 3" 6 outlet strip from
Walmart for our cell phone charging station, camera battery charger, and laptop plug in.)
We love going for a couple weeks. It gives you time to get to know CMs and avoid the frustration of big lines. And we never feel a trip is wasted by unexpected illness. We do take newbies with us sometimes so have for one week lived the "never enough time" vacation again. We always make sure we have several days after them just to relax.
Have fun! Disney at Christmas time is remarkable.