Check the GVWR of both trucks. If you are towing a fifth wheel, your cargo rating on the 2009 truck should be a lot more then the 2007 Diesel. Even if both have the same GVWR, the gas truck is about 500 pounds less than the diesel truck.
Someone will aks if this is a early 2007 or later 2007, I don't know the difference, but one is much better than the other.
Even though both of the diesels "can" tow 12,000 pounds, neither can handle the 2,500 pound pin weight of a 12,000 pound fifth wheel trailer and still carry more than 140 pounds in passengers. Both can handle the 1,200 - 1,500 pound weight of a travel trailer.
So it depends on if you ever plan on towing a fifth wheel.
Fred.