Should I hit 'reply' or 'comment on'

To clarify what LanX said: You use [comment on] if you want to reply to the top visible node of the thread you are currently reading. But you use [reply] next to a particular comment ("node", or what you might think of as a sub-post) if you want to reply to that node instead of the top node.

Whichever of the links you choose, the forum will show the node you are replying to directly above your edit-the-new-post window, so you know where exactly in the conversation you are.

So, when I am looking at your Re^6: Need to speed up many regex substitutions and somehow make them a here-doc list, I clicked on [comment on] to reply to your "Apologies" post -- and I can see the text of that post while I am creating my answer. (Once I [preview] my post, I lose that context and instead see the rendered version of my post above the editing box before [create]-ing it.) If I had wanted to contradict LanX directly, I could have clicked on [reply] next to his post in the threaded view (and in fact will be, soon).