I've had hard, soft and in between tonneau covers in the past from small to full-sized pickups.
My first few I tried and tested vigorously for MPG gains over various driving techniques. Never saw an MPG increase from any of them.
But that's [MPG] not the reason I purchased them for any of the vehicles. Security and general protection of bed accessories was always my number one reason/concern.
For the record, the very best tonneau cover I owned was a fiberglass ARE coor-matched on my 05' Ford Screw. Fit-n-finish, appearence, functionality and pretty much everything was just top notch. If ARE had a hard tonneau cover for my Rambox model I now own, I would jump on it in a heartbeat.