There is a difference. A comment is voted up at the discretion of the users, while text associated with the url would be displayed at the top no matter what. Which would mean the first person who submitted a link could essentially make their comment upon it always be the top one, and define the discussion.
I think you're missing the point. Imagine if everyone on hn posted a link that directed people to half of the content, and put the other half in a comment. That would make for a pretty crappy trend. Note that your original comment has 25 points, so as you can see there's incentive for people to start doing this.
I think a non-vote-able comment at the top wouldn't do as much damage as you might think. Maybe, instead, make it a rule where half & halfs get filtered/blocked.
