I probably will come off as a little jaded, likely because I am. Anyway my take on this is, if you are trying to impress elk calling elitists, the diaphragm is the only way to go. In addition the diaphragm is somewhat more versatile. If you are just using your bugle to call elk, both are up to the job and the reed is easier to learn. They are also less prone to making poor sounds if not used just right.
The whole thing reminds me of fishermen. Some say "fly rods are the only sporting way to go. That is what the true sportsmen go with." Others go with a casting or spin outfit and get as many fish.