This is one of those tough questions that each person will answer from his own experience.
My opinion is the same as txhunter. I have seen elk with a front shoulder busted and other elk with the rear quarter joint busted just keep on running I was amazed at how far they went.
A good double lung shot will drop an elk quickly, and a spine shot drops them immediately. If you only get one lung the elk can still cover quite a distance.
I am sure others here have seen elk drop by taking out the front shoulder, that has not been my experience unless you hit high in the shoulder and hit the spine also.