Weather can be variable. We have one Christmas trip and two January trips (New Years and mid-month) under our belts, and its been a mixed bag. Hot some days. Downright cold (wearing our winter coats and gloves) other days. Pools are heated so we swim regardless....we're Canadian, eh!
I'm a big fan of offsite. Its more peaceful, its WAY more spacious (you will get a 1200 square foot 2 bedroom, 2 bath villa with a full kitchen, laundry facilities, screened balcony, etc. for the same price as a 260 square foot Disney value room), and its far easier on the budget. Dining at Disney is very expensive, table service meals need to be pre-booked months in advance, and food quality does not match the price (the "experience", in things such as character meals, does make up for
some of the price difference but not all IMHO). I would stay at an offsite resort, eat select meals at Disney, then dine for the remainder of your meals at the plethora of options in and around the theme parks.
I agree with the PP.....Christmas week is beyond busy. If your DH isn't a huge Disney fan, Disney at Christmas has a good chance of turning him off completely. Crowds are beyond what you can imagine. Some parks will close very early in the day due to capacity.
The Christmas season itself is a wonderful time to visit, so if you can go earlier in the month, you should have a much more positive experience. You would be able to take in a Mickey's Very Merry Christmas Party if you wanted.
There's also some great offsite seasonal attractions such as ICE! at Gaylord Palms....super awesome and so festive!
SeaWorld is amazing at Christmas too, and their annual Christmas Celebration is not to be missed. You can have dinner with Santa and Mrs. Claus, have your photo taken inside a snow globe, watch some of their amazing Christmas shows, see it "snow" along the waterfront, be amazed by the Sea of Trees, ride the Polar Express (and visit Santa at the North Pole afterward), all in addition to their regular show and atraction line ups. The nice thing about SeaWorld.....all their holiday events are included in the price of regular admission ticket, so no additional cost (Santa's Fireside Feast, of course....and any food for that mater...would be additional).
When comparing your costs, be sure to factor in the "extras" of both onsite and off (like a car rental for offsite, and things like tips, extra snacks, a third meal of the day, etc. for onsite).
We have done both onsite and off, and even if the costs were the same, it would be offsite for us without a second thought.