PWM is better and a more efficient use of energy / power, when it is done properly.
- PWM uses pulsed power while variable voltage generates heat reducing the voltage from it's source 12v.
- PWM has the ability to run at lower speeds than variable voltage because it generates full torque every time power is applied (which can sometimes create the clicks and rattle).
The above are the reason industrial motors are now mostly PWM and not variabable voltage.
I agree with Dr Strangelove, PWM is only as good as the PWM controllers, but there are 2 control components involved; the one built into the fan and the one fan plugs into.
While computation noise can be a problem, the power pulses can be done so there is no noise .. be it in the bearings, fan blade vibration, power surge, etc. As Dr S says, it's not a problem if the start/stop pulse is rounded,
PWM fans will also run with lower than 12v power, which means they will run at a correspondingly lower RPM range.
I guess TY-147A is a one of these 'best cases', because it's PWM control is from just under 300rpm to 1300rpm.
The short answer,PWM is preferable, but it really on what your computer system can properly control .. and what what the fan prices are.