Naturally, the manufacturers of elk calls will beg to differ, but the truth is that the real difference maker when it comes to calling elk is your own vocal control. Even the best elk mouth reeds still need you to blow into them and vary your tone depending on what kind of elk call you are trying to make.
In that regard, an elk bugle is about the same thing as playing the trumpet. If you practice and play it properly, you make beautiful music. If you pick one up and blat mindlessly into it, you are going to scare everybody away.
It is far superior to be a great player on a poor instrument than it is to be a poor player on a great instrument. Whatever elk call you feel comfortable with will probably do the job nicely so long as you spend some time perfecting the way you use it.