Shorter brace heights can give you more speed but less forgiveness. For example problems with poor shooting form may be more noticable with a shorter brace.

Longer brace heights are the opposite. Little slower but more forgiving. In my opinion this is better for someone just starting. Also for people with long draw lengths.

Here is a good link with further explanation : http://www.huntersfriend.com/bowsele...braceforenergy