The difference is in the size over standard. Go with the .020 as it is closer to stock and will be able to usee the same carburator. You may need to rejet but should be fine with just .020 over. If you go .040 over there will be a noticable reduction in low end power and would make starting more difficult. You would also be near the edge of jetting for high end power and may need a larger carb to realize full top end power.
