Our former club president flew pattern for many years using an OS 61 FSR two stroke with a tuned pipe. When his 60 size Boxer finally died he bought a 2 meter plane and a YS four stroke. He was never happy with the plane or the engine, and eventually sold it to someone who loves it. He prefers the simplicity of a two stroke.

Our current secretary has won the Nationals and he only runs YS.

So, a lot of it really is just personal preference. If I were in your position I'd probably choose the AX. You already are familiar with the AX series, and it saves a bit of money. If you want a four stroke, a Saito 125 is a lightweight and fairly powerful choice.