A couple of things to note for me are I have gone away from any standard whistle (just the whistle) especially the Fox 40 and I have even put my mega in the drawer because of the sound pounding I get when I blow a whistle. Talk to several trainers who have been around a while and they will tell you blowing the whistle damages your ears. A lot of folks are wearing protection at the line now.
I have both the Blue Dallesasse and the Green Monster and they both use the same whistle (Acme Thunder). I have used the GM mostly because for some reason my dog responds to it better. It is a little heavier than the BD. Get both at least and work with the dog to see which works best.