I’m afraid I don’t have any great insights about when it would make sense to choose a decision tree vs linear regression. Decision trees are popular - at least in places like Kaggle. I suspect this is partly because they make it easier to visualize what’s going on inside the model.
There’s further discussion about this topic on this thread:
As far as your first question, in general, it’s better to make a new post for new questions. That makes it easier for mentors to notice your post, which means you’ll get a quicker response. It also can make it easier for learners to find the post in the future if they have the same question.