my question is which alarm is better a stock alarm or a aftermarket alarm??

 

most stock "alarms" are nothing more than a keyless entry system that beeps the horn after the second press of the lock button.

aftermarket alarms will have better sensors(door,impact or motion) and they do a better job of warning away a potential thief.

just remember that an alarm is a theft deterrent its not a guarantee that your car wont get stolen or vandalized.