I you want to compare, say, feature A in Asterisk with feature A in Vonexus (the only Windows based softPBX that I know of), then I understand the purpose of the comparison.
However if you want to compare: does Asterisk have such-and-such feature? Does Vonexus? then I see no purpose. You see, just take a look at the list of Asterisk features. Why would ayone on earth want a feature which isn’t there?
In fact, if they took several options off Asterisk, probably nobody would notice.
If you need a feature, you can always implement it (or hire someone, out of the thousands of programmers that know Asterisk or will know in the near future) using any programming language.
If mighty Microsoft doesn’t have enough money to counter the OSS folks, what makes you think that Vonexus will?
What you need to decide is whether you prefer OSS or CSS (Closed Source Software), once you make that decision all the remaining decisions: Apache or IIS?, Exchange or sendmail?, Asterisk or Vonexus? will just follow logically.