Their difference is in connector. One is PWM 4 pin which allow motherboard PWM control over fan, more accurate and generally should be better. PWM fan can run even at low speed like 300RPM, voltage control fans have problems with low speed. Other marked as FLX (this is just noctua naming) have 3 pin connector which allow only voltage control.
Otherwise those 2 fans are same.
So, which one is the PWN one?
Here are links to manufacturer site for both
FLX
PWM