Hi Aliweshky: although I would like to go to smaller PSUs (more around 150 W) and in general want to go to a lower powered (from an electrical point of view) HTPC, the Seasonic fanless looks decidedly nice: very tight voltage regulation and efficient and silent... but not cheap. If you actually can leave the PSU outside the PC box (lots and lots of cables to go from the PSU to the inside of your case....) and fully out of sight (on/under your table might be difficult) that is one option.
But I would prefer the PSU inside the box .... and take my chances on the electromagnetic field.
That would suggest the Streacom adaptor, certainly if you have the matching case: you can also email to the guys there, in case of questions, they are pretty responsive: support@streacom.com.
I would also be interested in the specs of that thing.... they do not give out much data on the website and do not promote it as a HiFi component, unlike the HDplex guys... but OTOH it provides a complete PSU. Streacom makes hiFi stuff as well, so hopefully they are doing something stupid in that sense...
I would kind of hope that it "just works", in which case you have a cheap, neat, compact, noiseless solution (and can start worrying about something else).
Like you said, the metal case, screwed directly to the case provides some cooling as well and shielding against electromagnetic fields inside your HTPC.
The linear PSUs are very nice, but huge bricks.... pretty expensive too at $358.- for 100 W.
Using the linear PSU with the convertor would probably loose some "linearity" again: ripple noise and output voltage variation are not nearly as good as that of the linear PSU itself.
My first suggestion was going to be: see if you can be happy with the Streacom from about 80 euros - you may find that a convenient and "good enough" solution.
If not: put it on sale and get another PSU solution.
But I changed my opinion after reading this review. If you can read German, try
http://www.computerbase.de/tests/ If not: use Google translate.....worth the effort or confront the support of Streacom with this article and listen to what they answer.
In short, this for the Streacom 150W PSU which turns out to something of little disaster, electrically speaking: no mention of distribution of power across the various voltage, very poor voltage regulation, high idle power consumption, runs very hot.... about the most destructive review I have ever seen, and yet very thorough!
This, added to the fact that Streacom still does not show any specs, makes me very wary indeed. Other pico converters just pass the 12V right through, so then much depends on the quality of the 12 V supply. see a comment on Amazon.com regarding this.
The linear PSU is by far the cleanest solution.... but 100W is probably not enough for you...
In that case I would go with a conventional high quality PSU, preferably fan less.