As silly as this sounds the type of people that make this decision are generally looking or should be looking for different experiences from each device.
Ph are hands off device with minimal user set-up where as HTPc's are a little more involved, or can be a lot more work depending on how you want to configure it or what software you will use, this shouldn't put any one off though as it is by far the most satisfying option and can be a lot more rewarding IMO.
As for silence its all down to hardware and case decisions. if you are building form scratch and building with this in mind then there is no reason why you cant build a silent HTPc, do you have an idea of the formfactor you would prefer to use or is this all undecided??
