A Ring cheekpiece introduces poll pressure as well as creating leverage on the mouthpiece, the 2 1/2 Ring version is the softest out of the 3 or 4 ring alternatives as the 1/2 ring softens the leverage on the mouthpiece and poll.
The Happy Tongue is a solid mouthpiece. It is curved and ported to give tongue relief. The Happy Tongue would be a good place to start if your horse is:

Shaking its head
Sticking its tongue out
Sucking its tongue back and as a result, making an intermittent coughing noise
Trying to put its tongue over the bit
Going behind the bit to avoid the contact
Snatching the reins forward and down

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.