Its impossible to know really whether either a real or fake alarm is effective as you can’t quantify what hasn’t happened.
There are some burglars who’ll take the chance even knowing a place is alarmed. When my workshop was burgled they just ripped all the alarm apparatus off the walls – inside and out – and smashed it all up. They’d have had to have done that with the alarm sounding – in plain sight of the neighbours and just banked on the sound of an alarm not being enough to make anyone look out their window.
What you can’t know is how many potentials opportunist an alarm system deters.
I’d say nowadays an alarm system that tells you that it has been triggered is more important that one tells your neighbours. Its no more or less of a deterrent but at least it allows you to do something to mitigate a break in if it happens.