Spiros @fiveten really explained this in the best way, but let me elaborate.
Headlines
An article on the Headlines page is a publication. You do it at a particular time, and can receive comments, and your post will hang around at the front page (for a while), in the Database, and in Google searches. It will also be present in your personal blog, which you and your followers can access through your profile.
This is why the Headlines should be used to show something or tell a story, but not for Q&A or WIPs. To share a milestone achievement with the World, but not the best place for your daily deliberations.
BTW. If you have something to add or follow up on the already published article, the best way of doing it is perhaps not commenting under your own post. Consider posting an entirely new "part 2" article and join the two to form a series, so that the users can freely navigate between the two. For example: https://imodeler.com/2019/11/1-35-pegasus-hobbies-moonlander-has-landed/
Groups
Starting a topic in the Groups Area means starting a living (and potentially lasting) conversation between you and your readers. Topics are meant to be living discussion spaces, and any new addition will bump up the topic to the top of the pile. This makes topics an effective vehicle for any discussion or process, for as long as anyone is interested in keeping on.
Your audience in the Groups topic may be a bit narrower than in the Headlines (no Google or casual visitors :), but also probably more dedicated to engage with you, and you will find ample space to discuss any items, big or small.
That's why Groups are the best places to ask for help, conduct a WIP journal, share your recent purchases, create a SIG, form a private club or run a Group Build. You will find many Groups made for these purposes, and you either start a topic in any of these, or choose to start your own Group.
Hope this helps!
PS. I realized that we haven't had a proper help article that describes the above, so I have added one now:
iModeler tutorial - Understanding Headlines, Groups and Database areas of iModeler