Personally,
I think the advantage that a ProTools HD setup with Farmcards has over a common SONAR w/PC setup is the stability that the HD setup provides when recording over and over live, and the advantage of instant Farmcard DSP power.
For example, I find an HD setup much more reliable if I am recording an artist or band, it is far more stable than SONAR in long sessions. That isn't to diminish SONAR, which is a great piece of software utilising native elements of a PC for its power, rather than Farmcards (which are great._
Farmcards, are a huge, albeit expensive advantage, and are what I like about an HD setup. I don't believe in the insanely high sample rate of 192kHz, but many use that as a benchmark, or a reason for argument.
Honestly, many people I know have bought into a cycle of planned obsolescence that Digidesign brings, myself included, and among peers that all must have the latest technologies, I have to keep up and know how to use what most of the studios I work in have. A lot of this investment costs a lot of money, and many people, may not have money. From the looks of this forum, many don't, and SONAR offers a great alternative.
Honestly, I believe, with enough time, that I could make equally good recordings on HD and CEP or SONAR if I put enough effort into it and have the right outboard gear and mics.
I think that high quality converters are what matter a lot, and a lot of people don't talk about it.
Sincerely,
God