As has been said, the lungs are bigger, but deer tend to crouch a bit when they hear the bow.
So, I've subscribed to the school of thought that says to aim at the heart/lung border.
Either hit will kill, but I like theidea of the heart pumping out blood faster with a double lung hit. That makes a lot of sense. Faster loss of blood = faster loss of consciousness.

Remember that bones are an archer's worst enemy, and while you can see the outline of the shoulder through the fur, the spine has to be visualized by just knowing where it is. The spine takes up the top third of the deer's chest, and is not nearly as high as many archers believe. If you hear someone talking about putting an arrow through "the void" in the top of a deer's chest, they simply don't fully understand that they can shoot over the spine, and still hit the deer. Check out the pics below. These are the best I have seen.

Since arrows do notdeliver the blunt trama and shock of a bullet or slug, rapid blood loss is the way they do their job. You are wise to investigate the difference in shot placement before flinging your first arrow at a deer. I salute you for being responsible.

-Bulz