You both are right (and wrong) to some extent. A .46 engine won’t turn a 12-6 as fast as a 11-6 so you might say that the “performance” is degraded. However RPM is not the only factor effecting performance. Thrust is. Southern touch99 is flying at a special condition (hover) where static thrust is the most important need. His choice of a 12 ¼-3 ¾ can produce greater static thrust. A larger diameter propeller with a flat pitch is usually a good choice for maximum thrust at static and low speeds. The advantage is lost at higher speeds where a smaller diameter and more pitch would be better.

All that being considered, the original question regarded propeller choice for a “trainer”, where absolute performance is the least of factors. Our little engines are remarkably flexible and tolerant of pushing a variety of props. For the trainer, just pick a prop within the range recommended by the manufacturer, and go fly. You can’t go wrong.