One is normally closed(Passive) and the other normally open. The Active is a higher quality and more accurate, however i believe the passive probe is still more accurate than the Tormach. If you are using it infrequently then the passive is a more economical choice, and what I purchased. Its very nice for the $.

As I understand it, the active wont need to have the contacts cleaned after thousands of actuations... think automotive points from old cars.

David