Anything you do behind the back, be it behind the neck presses or behind the neck pull ups, are going to place more stress on the AC (Acromioclavicular) joint than if you were to do them to the front. The AC joint is the joint that fastens the acromion process of the scalpula (the top process that sort of sticks out) to the clavical. Remember, joints are strong only under certain angles. The ACL of the knee for example, which we all hear about in sports related injuries, is very strong when it's subjected to stresses going up and down, such as walking, or squatting, and so on. However, if enough stress is applied from certain angles, such as a massive football player tackling you from the side, the joint is more likely to tear, because that joint, as well as most joints, aren't designed to be subjected to huge stresses under certain angles. They're strong from top to bottom, but weak from left to right. So apply that to lifting and you'll understand why strict form in neccesary for injury prevention.
However, changing the angles allows different regions of the target muscles to be more heavily recruited. As in, if you do behind the neck pull ups, you're going to be working the same muscles that you would be if you were doing pull ups to the front (upper back, biceps, ect..). However, by changing the angle to doing pull ups behind the neck, you're changing the angles of your muscles, therefore allowing for changes in muscle recruitment. The same principle applys to everything, with the best example being flat bench and incline bench. Both exercises work the chest as a whole, but by changing the angle as you do while doing incline presses, you're recruiting more heavily from the upper region of the chest.
Getting to the primary question; you're not going to see any major differences between behind the neck pull ups and pull ups to the front. Yes, you are going to change the muscle and joint angles enough to work different areas of the same muscles to a degree, but personally I think there are way better exercises to chose from for total back development. You are going to put more stress on your rotater cuff and AC joint, which don't balance with the pros of this exercise, in my opinion.
Just for the record, some of the back exercises I'm fond of chosing from include: pull ups (weighted), bent barbell rows, one-arm dumbbell rows, deadlifts, lat pulldowns, v-bar pulldowns, seated calbe rows, good mornings and hyperextensions (for targeting lower back). So my ultimate opinion is, for total back development, behind the neck pull ups are by no means neccesary.
Hope that was of some help!
