One other consideration is convenience.
I use to chase all over town buying tires from whomever seemed to have the best deal. Costco, KMart, Sear, NTB, Tiretown.
I finally realized that, for me, convenience counted for something. I now use the local Goodyear dealer. He's only 4 blocks from work, drops me off and picks me up, lifetime balancing and rotation, and road hazard repair. A small discount because I am a neighbor and my wife teaches his grand-kids. The balancing and rotation are enough to pay for the cost over the discount stores. Not to mention the extended life of the tires because I'm more apt to get the work done than with another store that might be 10 miles or more away.
Just a thought.