A no vote here. Have had both Theme Park and Lake view and believe Lake view is far better. First, the theme park view is mostly just an ugly parking lot view if you get any floor below 7 (5 and 6 are considered TP view and if you get them, you may feel ripped off by Disney's decison not to make more of the lower floors standard view which 5 and 6 should be). 7 is even iffy and you really need 8 or above. High floors are a non-guaranteed request. Moreover, from the high floors, TP view still has the ugly parking lot, a view of the monorail and the side of Cont. You see only a small porition of the lake and the theme park somewhat to the right with the closest view being the very unexciting top of Space Mountain. What theme park view is good for is seeing the castle (much farther away than many pictures make it seem) and wishes at night but not much else.
With Lake view, and particularly if you request and get high floor, you can get some incredible views like on the North wall where you see the lake and lots of trees in the distance, a channel, and to the left MK (the same stuff at MK you see from TP view), or the the end of the south wall looking at Epcot and seeing the lake and the nightly water parade. From the center of the building you get a view of the large lake, the former Discovery Island and forest in the distance, an excellent view all day long and not just for the 20 minutes of fireworks at night like TP view.
Also, by Dec I assume you mean next year and that you will have use of the 11 month reservation window. If you will be relying on the 7 month window, then the only week you will have a real chance for TP view or lake view or any room at BLT is third week of Dec.