I've divided sirens into 2 general categories: "discreet" and "obvious." These are based on whether or not a non-enthusiast would see a siren and be able to tell it's a noisemaker or not. For example, a "discreet" siren would be a Model X or a Screamer. Whereas an "obvious" one would be any siren that has horns or a horn (STX-10, XT22, rotational sirens, etc.).
Which style do you prefer?