With radios, you have to have someone listening with a radio on the other end, on the same frequency as you. I would just go with a whistle. Probably about the same range as an FRS radio in canyons/valleys, and nothing is required on the receiving end for them to be able to hear you (except ears).

That being said, I do sometimes use FRS radios on camping trips, when our family is separated (possibly fishing in different areas, etc.) But we are relatively close to each other and are all on the same prearranged frequency. We're not transmitting in the dark hoping to reach some unknown person by chance.