RE: Suggested Engine Size for Bi-plane
I have a ST 90 on my ULT built from a kit. It is a good combo unless you want to hover a lot. I run a 14x6 prop on mine. It needs 90% throttle to hover. I have heard a 15x4W is a better hovering prop for this engine. I just have not tried one. I have other 3D planes and don't use the ULT to 3D. Of the two engines you have the ST would be my pic. If I were starting from scratch the Saito 1.25 would be my choice.
David