I've used both the real silent and the GT, i can tell you that the GT is definitely quieter at the same speeds and cooling. Not sure on the corsair, but the nexus > the antec. i cant remember exactly, but the nexus beat all of the noctua 120mm that silentpc have reviewed (not sure if it includes the f12.)
that's barring stuff like warranty and longevity etc, but if you're after performance the GT is the way to go
I went with the Noctuas as they were -50% off during that weekend. Seemed to have been their most sold fan as they had 100+ of them when the sale started on Friday and they were all sold on Sunday morning.
I got them installed today (and at the same time removed that stupid metal piece blocking the top fan in my P183) and I must say that I'm slightly dissappointed at the performance. I currently run them at around 850 RPM which gives me around the same temperatures that I had with my previous fans but definetily quieter. Why I run them at such a slow speed is that if I go higher than that, around 900-1000 RPM the fans start to give away a low whirring sound, instead of the low woosh sound that you get when the air is moving. It's still pretty quiet at those speeds, but the sound is more annoying. At 1400-1500 RPM they are definetily not quiet. The end result is that I have a mixed feeling about them. On one hand they give me about the same performance and quieter system compared to my old fans, but on the other hand I won't be getting better performance at around the same noise level because of the more annoying sound. Which makes me want to try replacing all the fans with Nexus Real Silents instead but I've already wasted enough money on fans.