Short answer - there isn’t any.
Long answer - Many people who want to have t-shirts screen printed sit down at his or her desk and either use the phonebook or internet to look up a local printer. The logic of course being that local is better. It will be cheaper and I will get better customer service if I use a local printer as opposed to a national printer. While these are not an inherently false statements, the truth is that the printer, not the geographic proximity, is what is going to make your experience and final product live up to the high standard that you deserve.
The truth is that with modern technology and telecommunications the advantages to printing locally really do not provide any serious net gains for you, your customer, or your business. The internet has removed the need to ship logos or other designs to later be transferred to the t-shirts. You can just as easily e-mail some down the street as across the country. A printer in Manchester, NH and a printer in Seattle, WA can access the same information with the same speed and ease. The next question that inevitably arises is whether the quality that one would get from e-mailing images is as good as from shooting directly from an image. The answer is a resounding yes. In fact, sometimes the image can have greater resolution and clarity because nothing is ever lost in the scanning process. Often time local printers will ask for a digital copy of the image that they are going to be imprinting on the t-shirts. The important point that you take away here is that the internet has eliminated the local advantage one would receive from a custom service standpoint. The quality of the company and its workers is the defining aspect in the modern world. Good companies will far out way being able to visit the plant. Whether you can see the t-shirts being made or not is not what is going to make your experience with screen printing negative or positive. The caliber of the screen printing firm will be the single largest factor in determining your satisfaction.
The second aspect for the local argument is price. If nothing else shouldn’t save more on shipping? The answer is that if you just look at shipping, it will probably be cheaper, but as a wise consumer, you are probably more interested in the bottom line. You may pay a few dollars more for shipping, but over all your cost can often go down when using a screen printer who operates nationally. After all, if they couldn’t compete, then they wouldn’t a national company. Whether a t-shirt company is taking advantage of economies of scale or they are just less expensive, it still makes sense to use a company that is overall cheaper, not one that is local and will save in one area, shipping, only to make up for it and then some with an overall more expensive, albeit identical, product.
The quintessential point is that the location doesn’t matter. Go with the company with the best combination of customer services and price. Propinquity may have mattered for your grandparents’ generation, but if you make that the priority, then you and your business may suffer. Use the internet and find the best deal, if you find yourself unsatisfied the local guys will be more than willing to charge you more and offer what the exact same benefits. It’s your call