I'd get what you can get the better deal on, or if you like the ride and handling of the coils I'd go with that. There's only a 500 lb rear axle capacity difference between the two which is meaningless at the size trailers you're contemplating, and by the time it does start to matter you ought to be thinking about a DRW truck. Just my two cents, and I know it breaks from the herd on this site.
Mileage wise it really depends on the driver, conditions and terrain. 9 to 12 is likely depending on how fast you guys drive and whether you're in the western mountains or idling down a road in Florida. It won't be much different than it would with a 6500 lb TT - maybe 1 or 2 MPGs lower due to the extra weight and wind resistance.