HN also has a much stronger focus on entrepreneurship (understandably so).
The level of technical competence on HN is very high, and there's way less noise, so if you're a technically competent person with a business side (or if you're just interested in the business side), HN has a plethora of good for you.
As you note, the level of technical competence in the subreddits and threads that pertain to technical matters is higher on Reddit. Articles and discussions about just about any facet of any field that requires long-studied domain knowledge will have at least one commenter that really knows their stuff and wants to help people understand it.
This does come with the problem that someone who is just crafty with words can create detailed troll paragraphs that laymen can't easily distinguish from legitimate information (and this does happen, and is arguable inevitable once a discussion community grows to contain some ill-defined number of members), but that is a problem endemic to language itself, and it seems like at some level you have to assume that people know how to critically analyze what is told to them and be able to make a fair guess at legitimacy, and know to defer to well-sourced data (in addition to being able to identify good sources....)
The problem with groupthink and immaturity on Reddit is something that occurs in almost every large(r) online community I can think of (/. anyone?), and has many causes, not the least of which is that it's comprised primarily of young, idealistic, nerdy people -- and we all know that it's much easier to be loud and passionate and blunt-to-a-fault when you haven't gotten past whatever developmental phase it is where you get over yourself a bit and stop being so reactionary. In addition, you have group psychology and a large percentage of users that aren't the most socially well-off folk. In addition, a bunch of other stuff.
In short, it seems to me that Reddit is more a force for "good" than not. There are "bad" things, and the community can be swayed and manipulated like any other -- but they're largely more aware of that than other groups, and seem to be more willing to actually listen to well-reasoned arguments than others.
They also have the benefit of becoming a very popular site at a point in time where the internet seems to be becoming a major player in social and political action, but that's a different can of worms.