Higher pitch gives faster speeds in general. To go higher in pitch, you may have to go down in diameter to keep your engine working efficiently. Lower pitches, often with larger diameters generally provide more low end pull. This is assuming that the high pitch and low pitch prop are both in the proper range for the engine of course.

Truth is though, experiment with a number of props in various diameters, pitches, manufacturers, designs, materials, etc. until you find what works the way you want.