I would go with the 55 in that airframe. It is a little overpowered, but the 30 will be underpowered and will have weak vertical. It will fly fine as long as you are not into 3D. The 55 will give fantastic vertical but the GP Yak is not the best choice for 3D as the wing loading is a bit on the high side. Their is a good thread on it in the 3D forum.