I have a 7 foot Marathon Deck:
www.marathondecks.com
Truck: 2007 GMC CrewCab ShortBed 4x4 w/Air bags sitting on 275/65/20 Tires and 20x10 wheels. Edge Evo Chip, 4" exhaust(turbo back)/intake (for MPG measurement purposes its important to know the size of the truck, tires, chips, etc...)
15,000 miles on the truck since november, all with the deck on.
Deck is hands down easier than 2 place trailer. The ramp is super light. So much easier to turn, get into gas stations, drive through, park, etc.
The deck does grab more wind, but i still see better mileage with the deck than with a 2 place.
55-65mph: 2 sleds: 15-16 mpg 1 sled: 20 mpg
65-72mph: 2 sleds: 13-15 mpg 1 sled: 19 mpg
72-78mph: 2 sleds: 13 mpg 1 sled: 18 mpg
78-85mph: 2 sleds: 11-12 mpg 1 sled: 17 mpg
One section of a trip I was had the cruise set at 85mph and was running into a 45+mph headwind. with the airbags no sway issue, but i got 9mpg on that tank and destroyed 2 covers...
I will never go back to an open trailer again. The only think i dont like about the decks is climbing onto the deck to fuel them up. This can be eliminated by using gas cans stored under the deck.