OK I think the proper question would be comparing 12 x 6 to an 11 x 6, Each would move the plane 6'' on one rotation.
What does the extra inch do for me? ( decrease my RPMs? )
Steve
Yes, it will decrease your RPM by putting a greater load on the engine.
Tinkering is the only way to find the right prop for your airplane, engine, and style of flying combination.