In theory, a higher pitch gives you more speed at the same RPM.

However, you should try a MAS 9x4. The 9x4 will turn up to a higher RPM and might be just as fast, or faster than the 9x6.

Most guys use a MAS 9x4 on their combat planes for several reasons. MAS props are most common because they are the most durable.

9x4 is the most common size on .25 engines because you get high RPM and good straight line speed, and more importantly, you don't dump speed in the turns. Most combat pilots will gladly trade a few mph on a straight line for more pull in turns and vertically, since most of us don't fly a lot of straight lines in combat, and you have to turn sooner or later. So a 9x4 might or might not be as fast in the straights, but will certainly pull you through loops and turns much much better, and that's more important for most guys.