there is no such thing as a hardware version of Pro Tools. Pro Tools is a software program, not hardware. ALL versions just happen to have certain hardware bundled with it that is required in order to use the software.
the real reason Pro Tools became a standard is because they cornered the market a long time ago, and still pretty much do so now. Because you were required to use their hardware, the TDM systems were not only built very well, but they came out with processing cards that would relieve the strain of CPU usage. This was definitely important back in the early 90s when computers were still struggling. Even today, the number or I/Os available on an HD unit combined with TDM processing power, plus, for post production editors, the ability to be integrated with Avid systems, is pretty impressive....but it also makes it costly. But if you're going to spend several hundred thousand dollars or a million dollars on a recording console.....i'd say an HD system is chump change in comparison.