I am not a big fan of anything that controls your dog for you.
In the literature for the GL (or maybe the Halti, I can't remember, but they both work on the same principle) it likens the head collar "correction" to the corrections given by a mother dog to her puppies. Corrections given between dogs are corrections based in dominance, therby making all of your interactions with the head collar on a dominance action, not a training action. "Training" through dominance is not teaching your dog anything other than you can dominante it.
Your dog learns not to pull with the head collar on because when it does, his head is pulled down or to the side. Well, what happens when you switch collars and you don't have that aid? You go right back to the beginning.
Also, the collars stop the dog with a down and back motion that is likened to the way a horse halter works. Dog are not horses. A horse is taller, heavier and has completely different psyology. Their neck is long and supple and easily handles any pulling a puny human can dish out. A dog is shorter than you by a long shot, making the corrections go in an up and back direction - the exact direction that a dog's head is not supposed to go in. My dogs can hit full speed in a couple steps, I'm sure yours can too. And when they hit the end of that leash against your bodyweight you will crank their head to the side just because of the design of the collar.
According to GL they have never seen an injury from the collar. I personally think that is bunk. Moving a dog's head in a completely unnatural way is not not going to cause injury, especially if it happens quickly.
Though I have heard you can transition from the GL to a flat collar, the method of correction is completely different. You go from dominance to compulsion - which is not the same.
I have prongs for both of my dogs, though I rarely use them. You use a prong collar to teach, not to control.
Inside me is a thin woman trying to get out. I usually shut the bitch up with a martini.