RE: Heart or Lung shot (which is more ethical)
I don't believe one is more ethical than the other because ethics itself is hard to explain and means different things to different people.
There is no doubt that collapsing the lungs or slicing bleeding the heart results in a quick kill. Each deer is different to the quickness of this effect. I'd like to think that they are equally ethical for a quick kill.
As for aiming, yes the lungs are larger. I aim for the top of the heart. If you make a perfect shot and the deer doesn't move quick enough you'll take out the heart and the lungs. If you go slighly low, it's a heart, if your slightly high, it's still lungs, higher yet is a spine or flesh wound or miss. As long as your left/rights are on, the top of the heart is a good target.