RE: "50" size planes
I believe that there are 40 and 60 size planes... this 50 size is just a 40 size that they know you are going to want to stuff more engine into. The average 40 size plane weighs between 4-6 lbs and I would bet your "50 size" planes are in that range. It's just marketing for the most part. You will notice that the first feature of the OS .55 AX is that it fits the same mounting dimensions of a .46 AX wich also happens to match the venerable .40 FX. Now this of course is just picky semantics but to answer your question if you are going to play with some 3D then you will want the OS 55 AX on a "40/50" size aerobatic plane. I have one on my Hanger 9 Funtana 40s wich is just the predicessor to the Hanger 9 Funtana X50. They are esencially the same basic plane with a few nice redesigns to the airfoil but either would fly with the same authority when matched with the same engine.