I really don't think the nitro content will make that much of a difference.....I ran 15% when I flew glow in everything including 4 strokes.......When you snapped a prop I promise it wasn't from the nitro content of the fuel....probably from a defect in the prop.....I had a prop that failed after the first flight...cracked down the blade long wise.....
Run whatever prop you like as long as it is in the range recommended......
20% nitro will run a little hotter.....give you a better idle....I doubt you'd see the difference in top end