Hey Looneytunes, I have the lazy P-51. I used the OS- 40. It was also hard for me to decide. I am glad I went with the 40. The plane flies great. I can put the flaps down a little (dual aileron servos) and almost fly at walking speed with a small amount of throttle, then I can power up just before it stalls. It has great power for nice big maneuvers. But even with the 40 it won't go vertical very far. I like to back off the throttle coming down and power up going vert. As for the weight I put the battery behind the servos to compensate for the bigger engine. I did not add tail weight.