Yeah, i would say to go with the 7.3L with a manual tranny. If you are hauling some serious weight and want to climb the hills with the best of 'em, i would reccoment putting in a shift kit, tranny cooler and gauges just because that is some significant useage. My buddy who is head tranny mechanic at Ford (locally) says that there are two NO NO's to hauling; one is to haul in overdrive -
ever- and the other is to back up a trailer (mostly under extreme conditions like backing up hills, but to avoid having to back a trailer whenever possible).
I am sure that you would love it if you got one, but stick with the manual in the 7.3's. In the 6.0's I hear that an auto tranny is great for hauling (but no personal experience here)
~Turbotime