The sos whistle - an alarm continues sounding while you maneuver for escape, and frees both your hands for defense. And as you run away, an attacker will likely NOT chase after a noisy target.
an alarm costs more and has a battery that needs replacing occasionally.
All things considered, an alarm is your best choice – especially if you also have Pepper Spray as a backup safety device.
The best alarms continue shrieking until deactivated by reinserting a pin in a hole.
Always walk on the street or in parking lots prepared for action. On your keychain, keep pepper spray and an alarm quickly at-hand, ready for predators looking for easy prey.
(See Survival Options - Kidnapping for dealing with a predator with a gun.) In your car, with your keys now in the ignition, keep another unit of spray as a back-up within easy reach in your console.