The Drop Cheek has an Eggbutt like ring at the mouthpiece for the rein, with an upper cheek that has a ring at its end, to which the cheekpiece of the bridle is attached.
The mouthpiece does not slide on its ring resulting in only slight poll pressure. The cheekpieces lies flat against the horse's face, and the mouthpiece is quite fixed in the mouth and concentrates pressure on the bars.
The Happy Tongue is a solid mouth piece. It is curved and ported to give tongue relief. The Happy Tongue would be a good place to start if your horse is:
The rider will know immediately if this is the correct bit, as a horse with a sensitive tongue will finally begin to work into, and accept the contact. Changing the cheekpieces would then allow the rider to adjust the level of control.