I'm no dog expert, but I would think/hope that you would want a dog for more than just protection. If all you wanted was to protect yourself, I imagine you'd get a gun or carry around a knife. As far as buying a fully trained older dog, you lose all the time you would have spent with the dog as a puppy, training it, taking care of it, bonding with it. Getting a full grown dog makes sense if you don't have the time or the desire to have a companionship, but the way you worded your post makes me think that this isn't the case. That said, raising a puppy is a lot of work that maybe you don't want to do, but again, so is taking care of an adult dog.
In my (admittedly limited) personal experience, dogs that are protection/attack trained rarely have noticeable temperament changes, especially towards their owner. As my experience is limited to a few family friends who raised their puppies that way, I have no experience with the way a grown protective dog behaves and can't really comment on that.
You mention advantages to buying the fully grown puppy and the only one I can think of is the immediacy of protection, which forces me to ask how much danger are you in that that immediacy is necessary or are you just unwanting/unwilling to go through the training process with a puppy?