First, that seems like way too much money for both of those trucks.
Second, the 6.0 will get treeible fuel mileage. I have owned 2 of the 2007, 2500's. Figure about 8-9MPG driving around town, with or without a trailer. To me, the 6.0 felt weak when towing. Seemed like you really had to go pedal to the metal, to get away from a traffic light, without holding up other traffic.
Tough call on which way to go, but I still think they are both overpriced.