The working hunter saddle is a compromise between which two saddles?

It is a cross between a classic dressage saddle and a close contact jumping saddle - also known as a VSD or "all purpose" saddle. It is meant for flatwork and low jumps. These types of saddles in the USA have a bit of a more forward flap and higher cantle, while those made in England proper have a straighter flap and lower cantle. The English-made saddles are more true to the idea of the working hunter saddle and are generally of much better quality than those widely distributed in the USA. In addition, you will find these saddles referred to as "GP or AP" saddles in the USA, and as "working hunter" saddles in England.