I have the Seagull Edge V2 powered by a Magnum 91XLS with a 15X6 prop, and I really like it. I don't have first hand experience with the ST 90, so I can't recommend that prop for it. I will, however recommend against the 14X4W prop. That is a good prop for a plane with a nice thick airfoil like a Twist or stik or other fun fly plane. The Edge needs just a bit more speed than a 4"pitch provides with that size prop. You might try a 14X6 and see how the engine/airframe likes it. I don't know if your ST 90 likes to rev or turn a larger prop.

It's a good plane, I'm sure you will like it.