I'm still working on having them show up in both, but may I ask why you'd choose to discuss it in one forum but not the other? I'm struggling to understand the psychology behind it - for me, it makes no difference whatsoever.
I think there are three differences.
First, the News forum (at least how it shows up for me) is ordered according to when the threads were created, while the discussion forum is ordered in according to the thread received its last post. That ordering means that, in the discussion forum, interesting or contentious threads tend to stay near the top, while the News forum buries those threads. That's appropriate for a news forum, but not conducive to discussion.
Second, the discussion forum better encourages participation. If there is a particular point of an article that deserves separate discussion, you can create a new thread in the same space as the original, ensuring that folks looking for interesting community commentary find both. There even is (or was) a friendly competition to be the person to first post a link to an interesting article.
Third, inertia is significant here. The value of the forum is based on the number of other intelligent people looking at it. The fact that a community of often clever and insightful folks were checking and participating in the New Horizons forum was what made it valuable. (At least for me, this is the primary reason for coming to ENWorld.) When you try to move the eyeballs into another forum, folks adapt at different rates, reducing the value of the positive network effect.
I don't know how much the quantity of interesting discussion declined because of the forum change or because interest has died down more generally. (The level of interest from earlier this year was obviously unsustainable.) But I think it would be a real shame if this forum change resulted in a permanent decrease in the quantity or quality of discussion.