I have the ACR,Fox 40 and the Fox Micro (from a AMK PSP) and prefer the ACR.I had an upper resperatory infection and couldn't blow effectively on either Fox whistle but the ACR I could.In a survival situation you could be stressed,paniced,tired all of which could make you sick.
To me ease of use equals two thumbs up.Additionally their shape lends them to packing well in a kit...

BOATMAN
John