I recently purchased 3 Noctua NF-S12 fans, two of the 1200 rpm variety, and one of the 800 rpm variety. I very much like the reduction in volume. If I put my ear within a foot of the 1200s, I can hear a slight clicking sound. Any further away, and I hear a gentle whoosh of air passing through.
After listening to the NF-S12-1200s, I wondered how the NF-S12-800 compared. The clicking is a little more quiet, but not by much. The main difference is that the airflow is less audible, which is to be expected, considering that it pushes less air.
Also, if you add the Ultra Low Noise Adapter to the 1200s, they seem to be almost identical to the 800s, perhaps pushing a little less air. If I were to make the purchase again, I would just buy the 1200s, and if necessary, use an ULNA to mimic the performance of an 800.
The NF-S12s are quiet enough that, if I had the option, I might buy an NF-S12 that ran at 1400 rpm, and use the UNLA if I wanted anything quieter. I’m pretty sure that Noctua chose 1200 rpm for a reason, though.