A lot of the time local stores will have some kind of deal going on. A lot of the local stores around this area also include the price of mount and balance in the cost of the tire as well as offer something like free rotations for life.
The internet stores will sometimes have the best price, but by the time you factor in shipping, plus having to take the car somewhere for mount and balance the price can be a wash or just a bit more.
I buy off the internet, personally, as I like a certain brand of tire that only one shop carries locally. And I don't like that shop. I know the owner and I don't like him for personal reasons. I won't support him.
But that's one of the things to consider, too, when you get down to it. Do you support the local guy? Given that what kind of national warranty can be offered? For instance, are you buying tires from a local guy who carries Brand A type of tires or from one of the box stores that carries Brand B type of tires that can be warrantied at any store that sells Brand B?
I used to work for a local store that sold Delta tires. It's an "off-label" Cooper. Any Cooper tire vendor can warranty the tires. And Cooper has a large network of independent dealers (think Mom and Pop shops who offer tires for customer convenience, but don't make their money selling tires). But there's some tires out there that are available on the internet that no one sells out of a brick and mortar shop. Something else to take into account.