My understanding is the silicon bronze is a little softer than the manganese bronze. The manganese bronze is used in applications where higher temperatures are expected. I believe most OEM guides are iron (pretty sure that's what my Honda is). I'm sure there are special alloys (I've personally never heard of Moldstar) for more "serious" applications, but I can't imagine those being required for a club racer build. I think either bronze would be a good choice.