ok i need 9V to run 2 sound level meters and 12V/DC to run a neon tube sound display kitThose choices are quite dependent on a few things:
How precise and stable you want the voltage, ripple, etc.
How much current you need.
Stability of AC primary source does affect this.
That said, not everyone goes to great lengths to specify and thoroughly design a power supply. Often a simple supply is good enough - though the voltage and current limits do matter.
Share more about what you are doing so I can provide a better response.