King Arthur contracts with the major flour mills to make product for them; as far as I know they do not operate their own mills. While their specifications may be different for some of the products, they are being made on the same equipment. These same mills regularly supply very large customers who demand high and consistent quality product; their track records speak for themselves.
I've used King Arthur flours and many other companies's products. On a protein for protein, ash for ash, lb for lb basis, in other words comparing oranges to oranges and making minor hydration adjustments as necessary, I've found none of the differences to be significant or prohibitive. Pick a few products, learn them (like Liza says; the mfrs should give you the info you need compare) and run with them.
Recently, I've noticed King Arthur has been running promotions in the upper midwest and pricing their bread and all purpose flours much more competitively at the retail level. I'm guessing that they may have new marketing plan in place. Back east, their bread flours were (are?) reasonably priced in bulk at my local distributor.
King Arthur's customer service and education is unparalleled. They've trained a lot of small bakers and give attention to the serious home baker that you won't find from the likes of a Con-Agra. I like to give them business when I can, but if the freight is too much, there are alternatives--including a host of new local small mills.