Both fans have the same design, at the same RPMs they should sound similar, but in some cases motor might very, as 4pin fans are design to be pwm controled, and some dont like to be voltage controlled, like the NF-A14 PWM, in my experience it was unstable on voltage controlled header on an Asus Maximus VI Gene, they stopped after some time.
Now who will be more quiet, my bet is NF-A14 PWM, as it can be dropped lower in RPMs, as long as you are using a true 4pin PWM header. Here is couple of FanXpert2 charts, one from SPCR First 140 mm Fan Roundup: Noctua, Phanteks, Xigmatek
in there you will see the FLX version and the second is my personal testing with the PWM version on the GENE on the CPU_FAN header (true 4pin pwm).
Im my own setup, i find the NF-A14 PWM to be very quiet between 225-500rpm, noticeble around 650rpm, and loud above 1000rpms, so be prepared to run them lower than what they are rated.
But at the end i would chose upon how you will be using them, if its on a fan controller or voltage controlled i would simply go with FLX, and if its on true 4pin PWM headers (in most motherboards there is only 1), then i would go with PWM version.