Using a full face mask, such as the balaclava, doesn’t prevent fog build up in your helmet (although it does great in helping to keep the cold air from getting to you). This doesn’t mean you have to stop using one. It is possible to have the best of both worlds!
Above, we have a buddy of mine sporting one of his balaclavas and is also using a Fog Thief device. He only extended the size of the hole a little bit in his mask for his comfort and the Fog Thief had no trouble fitting.
Fog Thief can still direct the moisture down and away from the helmet to keep the fog away. Thus, you can enjoy wearing your full mask for warmth and have the Fog Thief improve your vision.
Why should you choose between the two when you can have both?