I got a Seasonic M12 II Bronze M520 a couple of months ago and I'm very happy with it. As far as I can tell from the reviews, it's identical otherwise to the non-modular version. The VX 450 is in my circumstances at least, certainly a lot louder than the S12; I know that from personal experience because I put one in the computer I built earlier in the year for my daughter.
However, it's not a like for like comparison; hers is drawing more power because there is a 5750 graphics card in there as opposed to mine which is using integrated graphics - cpu's are the same in both. The psu in my case (FT 01) draws cold air from outside the case where as her case is the more traditional type with top mounted psu taking air from within the case. I reckon I'm using 200W max so the Seasonic is not actually working hard.
Your perception of the two supplies is more about the cases they are in then the supplies themselves. Both the fans and the circuit boards are almost identical. Johnny Guru even states that the vx450w may be a S12II power supply with better capacitors. My NZXT Whisper
has a separate air path for the PSU
(and hard drives).
The VX450w powers an overclocked
Intel Quad-core, an AMD Radeon HD 5770
, eight 5400 RPM hard drives (quiet due to rubber and foam), two optical drives, and a SSD. At normal loads (100-200 watts) the fan does not speed up past the 600 rpm it starts at, so the VX450 is not at all noticeable.
(While gaming the PSU does spin up its fan, as power draw rises to about 300 watts.)