Obviously to really get an accurate comparison, you'll need to find someone who has hauled a hard side and pop-up on the same truck, same driving style, etc.
I do believe that the wind resistance plays a much larger part than weight as well. If both a hard side and pop-up camper are equal width, you're still interrupting the airflow over (and around) the cab in the same manner, and still left with a flat surface on the back side which causes a lot of drag. It's hard to believe that an extra 1-2' of height (all else being equal) is really going to equate to much difference in mileage. Hauling the hard side camper on my F150 only netted a 3mpg loss, even at 70-75mph, and I doubt a pop-up would be much different...?
2012 Jeep Rubicon Unlimited Six Speed - AEV 3.5", 35" KM2s on Argent Pintlers, Syergy steering upgrades, Ursa Minor J30 pop-up