The great benefit of brining is that it opens the fibers of the meat and allows the water, and what is dissolved in the water, into the meat. I suggest you convince yourself of this by adding a fragrant herb such as rosemary to a chicken breast brine, and comparing it side-by-side with an unbrined breast. The difference, deep into the meat, will be noticeable. It is not large chunks of the herbs that find their way into the meat, but the oils and dissolved parts. That is why you heat and steep the water before cooling it for brining.
I think (as is so often the case with preprocessed meat) it will often depend on who you buy it from. I often buy kosher prepared birds (which are always pre-brined salted) and I've never had any complaints. The nice thing about kosher preparation, is that it is done according to very specific rules, under strict supervision. It's not a factory process.
On the other hand, I cannot abide the brined birds from major poultry companies. Their primary goal is to increase the weight of the bird, not the flavor.
Factory prep seeks to minimize time and cost. They will use cheap ingredients (largely just water and salt, with no added herbs) and they will use whatever process takes the least time, most likely injection, which has the added benefit that they can force more liquid (and therefore weight) into the meat than would normally come from absorption. They'll likely insta-freeze it, so wait time is probably not an issue, but freezing causes its own issues.
I would say, for best results, take your time. A nice leisurely soak, real salt, and no pressure on the meat. For best results (talking turkey here) look for "Natural" or "Minimally Processed": if it says that, you shouldn't have to worry about competing additives.
Best Answer
For a brine, no it doesn't really matter, except that granulated sugar will dissolve more nicely than raw sugar. The sugar is for flavor more than anything else, though it does help the brine some. It also promotes browning of the meat. If you change one kind of sugar for another, I'd use a 1:1 swap by weight rather than volume so you don't have to worry about crystal sizes.
Though the sugar does help the brining action some, the salt is the important component.
From Cook's Illustrated: