Generally the original thread is the best place for follow-up links related to the same story; if we get another whole thread, the discussion generally won't be that different from the other thread and then we end up with a split discussion.
It also happens that a different URL is just significantly better than the one the ongoing thread was posted with; but in that case the most helpful thing is to let us know at hn@ycombinator.com and we can swap the URL on the existing thread.