My preference is quartering away. I say that because I haven't had a bear go any further than 50 yards with any quartering away shot. Even though you might only get one lung, you sometimes punch a hole through the liver, and most always punch a hole through the diaphram, getting one lung and the arrow usually blows out the other side, underneath the front leg (arm pit).
I heard a theory about why they expire so quickly with an arrow. When bears go to sleep for the winter, their blood pressure and heart rate drops drastically. When they take an arrow, their blood pressure drops probably even more so and they lay down as if they're going to hinernate and expire immediately. It's just a theory though.
DW