For 3D nothing beats a big 4 stroke. I have a similar type 3D plane to yours and it is awesome with a magnum 91 4 stroke. It has awesome straight out vertical power and torque rolls at a 1/4 throttle. In 3D, torque rolling at 3/4 power is a little underpowered IMO.

One thing I really like about the big 4 stroke is that fact that you can turn a big prop for huge torque. I am not sure what is the most used 3d prop for the OS 55 but I bet its not going to turn a 14X4 like the Saito 82 or the 14X6 that my magnum spins. That low idle with a big prop is great for 3D.

Before I would spend 150 bucks on a OS55 (which I think is WAAAAY overpriced) I would look hard at the magnum 52. $60 cheaper and similar power. I know you mentioned weight... but for the money the magnum 4 stroke is great and it doesnt need expensive high nitro fuel. I got mine on sale for 149 last year and it was by far one of the best rc purchases I have made.