Which is better and more concealing: the abaya worn over the head or the one worn over the shoulder with a loose, thick khimar that covers the chest and extends to above the knee?

The purpose of the hijab is to cover a woman from those who are not lawful for her to look upon. Any characteristic by which this covering is achieved in a lawful manner is sufficient. The two types of clothing mentioned fulfill the Sharia's objective of covering, and the preference between them is based on custom and common practice, not on a religious preference for one over the other.