Agree with Devon on the rubber happy mouth, but I have researched bits quite a lot and the thicker the mouth piece the more uncomfortable it can be due to the horse having a thick tongue and a narrow lower jaw/bars. I personally find double jointed, sweet iron with copper bits to be great on the sensitive mouth. The sweet iron technically rusts as it works, causing the horse to salivate and keep them soft. The copper quickly warms and has a similar effect to the sweet iron. I hope this helps.