I've no doubt, double lung every time.

With double lung, you have a bigger target area being more forgiving of a slightly off shot. Take out both lungs, and the deer is gonna suffocate in short order. Plus, his/her heart will keep pumping, leading to better blood trails and quicker blood loss and death. In my experience, if you take out the heart, while the deer is certainly going to die, it can run much farther and will not produce as good a blood trail...