I have both 210 1/2 and the adjustable. I had to adapt the adjustable whereby i put a piece of rubber tube on the end so i could grip it, as i found holding the whistle in my mouth for a period was a pain in the ass.
I have stopped using the adjustable silent whistle, i just find it easier with the 210 1/2. I trained my cocker with both whistles at one point and didnt see much difference in her response to the whistle.