Answer
Ford diesel truck. suburbans are known for their dismal fuel efficiency (8-15mpg depending on year/condition). while still not great on gas (being a full size truck), the diesel will be better for a couple of reasons: it weighs less than a suburban (by how much depends on year/body style), and diesel engines are significantly more efficent than similar sized gasoline engines (the ford will probably get somewhere around 15-19mpg). more info can be found on the mileage of exact years/models/engine sizes at www.fueleconomy.gov