I'd be inclined toward the 13" diameter prop. I flew a 46FX powered profile this summer on 13x6 and had mucho vertical on a 5 pound profile. Bigger diameter will help you with braking on downlines, and you won't need the speed of a higher pitch prop. With the 50SX maybe even a 14" prop, if it will have enough ground clearance.

The other thing I'd mention is to make sure you can get your idle down very low for landing in light or no breeze. First time I tried to land my Venus there was no wind to speak of, and the plane absolutely would not get down through ground effect. I had to take it up and run out the tank and then deadstick it in. After that I learned all about Idle Down and Throttle Kill on my Futaba 8U.