Either will fly your trainer fine. Fuel consumption will not be an issue with the FX. FYI OS has replaced the FX with AX version.
I know your desire to get a good engine to use in a different airframe when you are ready. I had the same problem.
Both engines are dependable and will fly your trainer fine. Obviously the ball bearing will have more power.

So it basically comes down to this:
Which engine are you willing to risk damaging when you are learning.

Also, your first engine will not be you last. You will end up having many more I can assure you.

So I'd recomend you buy a bushed engine, and save your money for a better engine once you've mastered your trainer.

And although OS manufactures a good engine, there may be other brands that are just as good or better for the money. But if you have a club helping, you may wish to begin with what they prefer initally.