IBrakeForNobody,
Between the two motors you first mentioned I would suggest the OS, but that's just my personal preference towards OS motors. Now for the other worry, the remote needle valve. I love the remote needle valve. On the older FX motors the needle valve mount was part of the backplate of the motor, every time you crashed it was guaranteed to snap off and cost you $40 like you said. But with the AX motor they got smart, they made the mount out of metal that is bolted to the rear of the motor. Because of this it can now survive a crash, the mount may bend but can be bent back into shape. And if it does break you can fabricate a new one yourself. This make the 46 AX a great motor IMHO. The only thing you really need to remember about a remote needle valve motor is to wait a second or two when adjusting it, it will take a second or two for the change to happen. As long as you remember that you will love the remote needle valve. And as said above, it gets your fingers away from a spinning prop.

Now for your exact situation, I would recommend the OS 46 AX. Here's why. As I said above at remote needle valve problem has been fixed. The 50 SX might be bordering on almost too big for your trainer, and for the cost difference the 46 AX would be a better fit. The 46 AX has tons of power and there are a lot of planes that will fly great with it when you get ready to move on to other planes.

Hope this helps