It’s not perfect, but HackerNews seems to have one of the most intelligent pool of users I’ve had the pleasure of interacting with. It’s not perfect, of course, but the discourse on a HackerNews thread is light years ahead of what you’ll find on Reddit.
With HN it depends on the topic. With Reddit it depends on the subreddit. Both have their high points and their low points. If you take the topics for which HN is very well-behaved and looked at their equivalent subreddits on Reddit, then you'll find they're likewise well-behaved.
In both cases you know exactly where the brush fires are.