Lungs are around the size of a football where the heart is more like a grapefruit. The heart lies LOW in the cavity and while deadly is a small target and easy to miss.

The heart and lungs both kill just as quickly. However, a heart shot deer will run at a much faster rate than any other wound and thus farther [though they will still not go far]. Additionally, I have ALWAYS found the blood trail of a double lung shot is MUCH better than the heart shot.

I'll take a double lung any time.