Here's a FAQ from Noctua on selecting among their 120mm versions:
http://www.noctua.at/main.php?show=which_fan_is_right_for_me&lng=en
The tradeoff is speed versus noise. High speed fans more more air, but are louder.
The F version is the highest speed version and is only available in PWM format. It would be ideal for CPU cooling, but would work fine as a case fan IF you have automatic PWM speed control to slow the fans down under low load conditions.
The P and S versions are slightly slower fans, max of 1300 rpm (P) and 1200 rpm (S). Both come with Noctua's low speed adaptors to tailor the speed if you are running them at a fixed speed. Both come in PWM 4-pin or fixed voltage 3-pin versions. Which of those to get depends on the specfics of your fan headers.
Personally, I would buy the PWM versions if you have any form of automatic speed control. The PWM versions work with 4-pin PWM headers and also work fine with variable voltage 3-pin headers, so they give you the most flexibility.