Choose the 12x6 if you want a climbing prop. Short takeoffs and the ability to go verticle.

Choose the 11x7 if you want a high top speed.

3-D planes may choose something like a 13x4 for hovering and the ability to accelerate out of a hover. Top speed will be limited just like a car that is stuck in low gear. These props also brake well when the throttle is closed. They brake so well that you often have to land with the throttle open a little so the plane doesn't just stall and bounce on the runway.

200 mph pylon racing planes might use something like a 7x9, 8x9, or 8x10 or something close to that based on a bunch of eperiments. These guys often carve their own props.