For competition, you want the 75B. You can use it as a single-action and thus get around the long, heavy first DA shot. The steel frame is heavy, which reduces recoil, and allows faster shooting. The sight radius is longer to allow more accurate shooting.
Differences in mechanical accuracy will be negligible between the guns. However, a heavy gun with a long sight radius that can be used as a single-action will be superior for competition or casual range shooting.