Physics class I think, aerodynamics and stuff. Anything that helps wind resistance will help as long as the overall weight doesn't offset the difference.

I had a soft top on my last truck and it was great. Roll it up when the bed is needed and roll it down when not. Keeps stuff dry and a below the bed plastic tool box is out of sight and light enough to slide out on the tail gate when needed.
I have a solid top now. Looks good and picked up for $150. so what the hell. Pain if I need to haul something big, like the motor I need to put in it now. Need help to remove it.