I've taken one full course and listened to intros on others. Also, I've been asked to join others.

I don't want to pick one over another, but I think the key is to keep writing, see what articles do better than others and focus on that niche. When you look at your stats, you'll see what they are. If you don't have a niche, I'd recommend it. Not every article has to be on that niche, but it helps.

Just as you spend time writing, spend time on finding the best image, and focus on your headline.