Maybe I misunderstand what's being asked.
There is no difference between fixed gear, single speed, and multiple gears with regard to pedaling in a given gear ratio. The various drivetrains might affect your riding habits, for example, choosing a lower ratio as you approach a climb or choosing a higher ratio to stand up and sprint if you have multiple gears, and deciding to coast on a descent or flat if you have single speed, vs. fixed gear where you of course have no choice other than to take a higher or lower cadence and to stand up or sit down to pedal.
If you are comparing three bikes with different drivetrain types but the same size chain ring and cog, when there is any positive force going through the crank (that is, rider applying force to the pedals, at any cadence or body position) any differences will be completely psychological/imaginary. Very minor differences exist in terms of drivetrain efficiency, or power robbed by friction within the drivetrain. These are too small to be significant in real world use.
I think you're asking about how a FG drivetrain affects a rider's habits or performance when the terrain tempts him to ease up on pedaling. There is certainly a difference here. But when standing up and applying power through the crank (whether it's 50 watts or 500), FG vs. SS are exactly the same.