1. This is the realism thread. So think realistically.
Real horses are intelligent and respond to their master or those familiar to them more readily that strangers. At the same time, domesticated/trained horses may respond to any human, depending on its disposition. So in light of this, I think horses should respond to master whistles over stranger whistles if they are both whistling at the same time, or if the master is closer.
2. If you know how to whistle through your teeth, or with your fingers, it is fucking LOUD. You can hear a whistle like this 100 yards/meters away easily if there isn't a ton of other noise around.
So, I would give horses preference to master whistles, but not exclusive to master whistles only. Also, I would keep whistle range the same.