Gas vs. diesel had almost no bearing on our decision. There were 2 main factors:
1) Cab comfort. I can't stand the cramped footwells of the Ford chassis and the Chevy isn't a whole lot better. And, the Sprinter chassis is the only one where I can put the driver seat as far back as I want when the slideout is in.
2) The Sprinter rigs a a bit narrower, which I prefer for driving narrow roads.
I sat in the new Dodge (Fiat) chassis. Can't put the driver seat back far enough.
As for mileage, I consistently average 16 when towing the car, 17 when not towing. This includes 6 trips over the rockies towing, 2 trips over not towing. Over the short term it varies wildly based on up/downhill and wind. I never drive over 60. Going faster has a big impact on the mileage. The biggest Sprinter fear I have is the risk of very expensive repairs.