I have to agree with Jamross. I started using Sika calls about 20 years ago, and found out about the Acme predator call from Gregor McLeod as they were flying off the shelves in his shop at the time.
I have used this call many many times, and as a rule it is the older stags that come to it. Like Jamross I have had them come at all angles when the rut is on, and if you are on a fire break they normally come through the darker timber to find you. I always feel they have the advantage of looking out into the light to look for movement.
The Screree Elk call I could never get the hang of, but have heard that it does produce good results. They are pricey I feel, retailing at about £20!! where as the Acme is about £5.
Good luck
Sikamalc