Head or lung shot isn't really the issue - it's about how quick you bleed the dead animal out. You really want to get as much blood out of the animal as possible (ideally - all), as quickly as possible. You'll get dark spots, unpleasant (metallic / iron) taste, and the meat will spoil quicker. It can get a smell too once the plasma starts to separate in the blood.
The red/pink juice that comes out of cooked meat is not blood. The blood is thick and slippery. Even if you can't get your animal out of the bush until the next day, try to do everything you can to position it so that when you sever major arteries and veins (usually in the neck, and when you remove the heart and lungs you'll hit the major ones too), the blood flows out and away from the meat.
This applies to fish, upland birds, etc etc. - anything you kill to eat.
Getting back to your original question - yes, a lung shot will cause more blood loss, and it's a larger area, and easier to hit, but I believe it's what you do when you're field dressing that makes the big difference in the quality of your meat.
Hope that helps