Andy,

I buy the hobby store out of APC props in a day!

Greg, I would also recommend the APC 11.5x4. I only fly Thunder Tiger .46's, but they are not that far off from the .50 SX. My Pro .46's scream with that prop, but transition is a little slow. On your .50 SX it should be PERFECT! I would just go out and buy all the props mentioned here and try one after another in the air after you've gotten used to the airplane.

Please let us know how that airplane flies...