knowing not only how to use cow sounds and bull sounds but knowing when to use them is what matters imho. i'm still trying to learn all the different elk talk and when to make certain sounds myself. sometimes i confuse myself :) timing is everything as i'm finding out. make the wrong sound at the wrong time and the gig is up and you just ran him off. i think #1 is learning how to use a mouth reed just because of the versatility a diaphragm brings to the table. i have ElkNut's dvd's and PlayBook that go in depth and i have killed several cows and a spike using his techniques. now granted i was trying to kill big bulls at the time but i am an equal opportunity bowkiller and when i have had golden opportunities to kill cows i've taken them. not bad for 4 years of hunting elk with a bow so far.
I don't buy into the "this brand or that brand is the best" theory. If you can use a diaphragm turkey call then it should be a cinch to call elk with it. All you really need is a echo tube to go with it (that can be as simple as a foot long piece of 1 1/2 inch PVC pipe). You can pick up an instructional tape at any sporting goods store that will show you the sounds you need to copy in order to call in bulls. Bugle sounds do work but not always. Bulls aren't always looking for a fight so I only use a bugle as a locator. If I can get an answer, I try to close the distance and switch to soft cow calls & chirps (bulls ARE looking for cows). Keep it simple. Over calling will lose a bull more often than not.
One brand over the other, I don't think so either. Wayne Carlton likens it to fishing, "you don't just keep throwing the same lure, you try different ones until you find the one they like." Elk are very similar in that you have to try various cow calls and bugle sounds to get them to trigger. I carry about 8 different cow calls, three differnt diaphraghms, and the E.L.K Power Bugle so I can try different sounds to get a response.
Elknut was mentioned and his book and videos are very good. I have multiple other books and tend to like Mike Lapinski's books and techniques, and you can't leave out Dwight Shuh's books. Read them all so you have their tools and experience in your toolbox.