I think the new format needs have its labels modified to make it clearer which “reply” button is which. The former is more appropriate when one wishes to make a general comment to a topic. The latter should, I suggest, be used with more discretion.
If one replies to a post, the whole of that post is displayed, and the new reply is shown beneath it. It becomes very tedious and consuming of space when a lengthy post, sometimes more than a page long, is replied to with a single sentence. I would encourage members, if they reply to a post, to trim down the original post to a sentence or two containing the main point they want to reply to.
Perhaps this point could be clarified in the user guidelines?
If you want to make a reply to just a portion (or even just one sentence in a post) - highlight the portion you want to quote - you should then see a (very) small box that says “quote” if you click on the “quote” it will place that portion of the post only in the box where you type your reply.