Assuming the two props load the engine the same and wind up turning the same RPM in the air, the 12x6 will give you better verticals and quicker acceleration for takeoffs while the 11x7 will give you more straight line speed and better wind handling. That's the simple answer, but in the real world the matter how the engine unloads in flight will be different with the two props. One will feel like its making better use of your engine's power curve than the other, so experiment with it.