The 120's give you a lot more speed options and at a given 'voltage' are quieter than the 140's but that's because they move less air. I would say that at a given 'airflow' the noise is more or less the same for both of them.
I guess it comes down to aesthetics and practicality and how much airflow you want to yield. I don't think either gives better performance per unit sound if that makes sense.
So far i've had a full set of Performance Editions (2xSP120PE, 4xAF120PE) a full set of Quiet Editions (2xSP120QE, 2xAF120QE, 2xAF140QE) and now i'm running 4xSP120PE PWM's with one AF140 in exhaust basically because it looks nice. The PWM's are badass because it's like having every single fan combined, and I would know i've had all of them! They go down to around 750rpm and up to around 2150rpm so they are pretty much the quietest and the highest performers. I've heard the SP120QE PWM's can go even slower so I may have to get a set of those next. PWM's FTW!
JR