Welcome aboard, Jason! There is no 20" bull barrel, just 22" and 24", the 20" barrel is a superlight, very thin contour barrel. For best accuracy and colony shooting, the 24" would be the best choice, but the 22" bull would work very well too, just depends on how much carrying you're gonna do. Even the standard contour barrel is a good shooter, the bull barrel won't heat as fast, so it has some advangtage in that respect.
Tim