I prefer the lung shot, and if I get the heart in there too it's a bonus. Where I hunt elk those things are always moving, and nobody every took a shot past 150 yards, and they are ususally around 50-100 yards - there is just too darn many trees.
I have yet to take a head or neck shot for the aforementioned reason, but probably should have in 06 when a cow elk presented a broad side shot to me. Huffing a puffing up hill, she's there between two trees about 100 yards uphill (what else!) I can see her head, and her body. I take the body, she drops... and gets up and runs off. This was 11 am. We track her all day, but lose the trail in the dark. First light comes and we pick it up again, only to lose it 3 hours later. I still don't know where exactly I hit her, but i'm sick about it to this day. No shot is fool proof, and never underestimate their will to live. I got one the next day with my .308, with a h/l shot that fell up hill towards me. She was dead before she hit the ground.
Same year a hunting buddy shot a herd cow 3 times in the h/l area and she kept running, to this day we talk about how seemingly impossible it was she was even alive, let alone running. Her insides were jello, and could barely distinguish any lung tissue at all, and the heart was blown to bits, as were both front shoulders. Only a head shot would stop her.
This last year, my son got his first elk with a shoulder shot that missed the h/l, but the 7mm mag 160gr Fail Safe creased her liver. He failed to consider the angle (which I need to work with him on). She was quartering towards him, and still dropped. You could tell she was in pain, and my 10mm headshot ended it quickly.
I killed two this year, both with double lung shots, both were running full steam. One was clean, was was not.
The h/l is not the end all shot, none of them really are, but it is the most forgiving, as many have stated. I would only try a head/neck if the elk wasn't moving, and was un-alrmed, holding still, etc., that hasn't happened yet. h/l is a BIG target.
Tom