I bought both at the same time, the trainer recommended the referee whistle but i thought i'd try out a proper dog whistle - I got one that was supposded to be best for German Shepherd types, think its the 211.5 but for some reason she treats it as optional, even though i used the same meathod to 'condition' it. First time i blew the thunderer she came bolting in from the garden to see what the noise was - i piled boiled chicken into her mouth and the rest is history! Though you really have to watch your ears!Interesting! Maybe the whistle I got for Zac was too quiet. He never paid all that much attention and when I lost it I didn't really miss it. It was an Acme but one of the dog whistles, maybe I should try again and this time use a thunderer!