By complete coincidence, I bumped into a gamekeeper last night who has 23 working spaniels!
Pete had recently recommended the Acme GunDog whistle to me so out of interest I asked this gamekeeper what he uses; He uses the Acme 211, but said that all the Acme whistles are good and, like Pete says, it's largely personal preference. If, as Pete suggests , you are able to 'try-out' in a shop then even better.
PS I ordered a Acme 211.5 last Fri and am awaiting delivery! Will let you know how the dog likes it in due course!