I don't buy chickens that much anymore, mostly when I see wings or thighs on sale. I seldom check the brand because generally there is not much of a choice.
I read an article about 6 months to a year ago which I believe I got from the Americas Test Kitchens website. I don't remember the contents that well except some of the more common names, maybe Tyson, did not fare that well. Is there that much of difference in quality in chickens as there seems to be in other products.
Also, just curious, do you buy the skinned boned breasts or breasts on the bone and debone it yourself. If you are not in a hurry the price difference is usually significant. The first couple of times I tried this, the end result looked like the chicken was a participant at "Custer's Last Stand" but now it is pretty easy.