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 (definition) 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 (definition)'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.