I'm not so sure the actual frequency matters that much. In the field, wind is often the biggest single factor with sound signals. Even with just a slight breeze your whistle will reach much farther downwind than vice-versa.

IME a Fox 40 is pretty good as far as a survival whistle. Every bit as loud as many supposedly better (read: more expensive) designs. Also, it's compact, lightweight and basically foolproof as long as you don't try to run it over with a Hummer or something to that effect.