The question is which engine is producing more thrust. The answer is neither. Thrust is produced when the prop converts the rotational energy of the engine into lift (forward, but still lift) along the prop blade. The total thrust produced is the integral of the lift function along the entire length of the prop. Therefore, it is easy to see that at a given rotational speed, it is the prop diameter that determines total thrust. Increasing the engine power will result in a higher peak RPM for the prop, which means more thrust will be produced as speed increases.

Another point to be made is the diffference between thrust and speed. Maximum thrust is produced with the airplane not moving through the air. As the speed of the plane increases, drag increases along the wing and air frame. At the same time, due to the relative motion between the prop and the air stream, thrust is reduced. At some point, where the drag on the airframe equals the thrust produced by the prop, is the constant airspeed of the plane (see image below for a graphical representation)

Now in the real world, we don't change engines. We change props. For a given engine, increasing the prop diameter for the same pitch increases the load on the engine, which results in a lower peak RPM. Depending on where your particular setup is in relation to the engines power curve, this may or may not result in an increase in thrust. If you reduce the pitch of the same diameter prop, the engine RPM will increase. This will generally result in an increase in the static thrust of the prop, but will probably result in a lower top speed of the plane, again depending on how much the RPM actually increases. Normally, the objective is to keep the prop loading at some optimum value (I normally prop for a peak RPM on the ground between 12,000 and 13,000 rpm) If I need more thrust (better vertical, quicker takeoffs), it will increase the prop diameter by 1 inch and reduce pitch by 1 or 2. If I want more speed, I'll decrease prop diameter by 1 and increase pitch by 1 or 2.

Brad