High wing / Low wing, is really the discussion of antiquated designs.
Designers trying to squeeze speed/range/efficiency...doing the most with the least amount of engine and fuel, will end up with a Piaggio Avanti type of design.
I'd get writers cramp discussing all the advantages, but the rear wing being higher then your typical low wing has the advantage of gravity feed, more ground strike clearance, engines getting better inflow and FOD protection.
When they start slinging Williams on the Avanti, about the time that fuel hits $9 a gallon, I suspect even the airlines will start pushing Boeing to reconsider where they put the wings...
Most of the supersonic designs on tap, looks like a Delta with a canard.