Well, the NV Engines GX 40 is .40 cubic inch displacement or 6.5cc. The Evolution 10cc engine is .61 cubic inch displacement. Thus the Evolution engine has quite a bit of a edge in performance due to its larger displacement. So the Evo 10cc engine is more powerful than the NV Engines GX 40. But the GX 40 acts more like a .45 or .46 glow engine in actual useage. I am running a 12x4 prop on the engine in my Hot Knife stunt plane. The GX .40 does well with a 11x6 prop too. Now then the Evo 10cc engine likes larger props as well as the 12x6 and 12x7 props work well on it even some 13 inch props work.
But GX 40 shines when you need a gas engine to fit in a plane where the CD module and extra battery won't fit. Such as my Hot Knife profile 3D plane depicted earlier. There isn't any place to put the extra stuff. Now if the plane has the room to stick in a CDI unit and maybe a extra battery, then the Evo 10cc engine may be the way to go. Also if the plane is a larger size plane then the Evo engine would work better as well. But if you had a smaller ,40 size plane the NV Engines GX 40 is the way to go.