If you will have a car add about $45 per day to the rate to cover the S&D resort fee and parking fee. If flying, add abut $25 per day and the cost of a taxi, limo or uber to/from the airport.
With discounts, the S&D are still usually significantly less expensive than the BWI. We often choose the S or D partly for that reason. Plus, the S & D have very comfortable beds. Be aware that the Swan has queen beds but the Dolphin has 2 double beds in their rooms. Also, the Swan has the first pick up and drop off bus stop.
Weather in Dec is hit or miss. pools are heated so you can use them anyway, but the air can be quite cool, especially if you are wet. WE don't like the clown pool at the BWI, we actually prefer the S&D pool.