I think the 50 would turn a 13x4 pretty good. Guess it kind of depends on what type of flying you're doing. For hovering around I would rather have the 12x4 on a 50. I'd be a little concerned with overheating on the bigger prop in a sustained hover. I'm not saying it would but would be more of a chance. Throttle response should be better with the smaller prop too. Hey, I may be wrong about it all!

Wade