Frame stiffness has little to do with frame material and a lot to do with frame design, although due to the increased chance of fatigue failure with AL, they tend to be built quite stiff. A stiffer frame also means a less comfortable ride, something that can be an issue on a longer ride. There are a couple of reasons for using a newer frame over an older frame, less issues with fitting newer components, which is something you may want for a long haul bike, you may be able to fit wider tires to a frame like the LHT. No reason why you couldn't build up the Super Course as a commuter with moustache bars and then get an LHT frame for your touring and rando.