HN
HN is wonderful in that you're more likely to learn something from the comments and have an intellectual discussion. If you can get to the discussion fast enough. Or if that thread doesn't magically die off. Due to the lack of a notification system on HN, there are lots of dangling, empty comments.
The problem is that HN is very much "all or nothing". If you post an HN question, you're either going to get 200+ comments with people discussing the minutiae or you get a 0 comment thread going.
Sharing links on HN is the same way. 90% of the userbase stays on the front page without even clicking "next" on the bottom that means that there are 20+ threads at any given moment but they usually stay static for hours at a time, giving no other threads any attention.
The upside is that self-promotion is totally okay. No one gives you shit for posting an article you wrote.
I don't personally spend too much time on /r/programming, but I do spend a ton of time on /r/webdev. The issue with Reddit is RULES. There are tons of rules, there are ruthless mods, and often, good content gets buried while terrible content makes it to the top. There's an inherent mistrust in the system.
With that said, I found it more likely to get an answer to a question on Reddit than on here. And I found it more likely that people would discuss something I post. Basically, Reddit feels like there is more small-user generated content rather than big journalism taking over. For example, check the top links for HN, how many of those come from big newspapers/online magazines that everyone knows? Now check Reddit, how about that ratio instead?
Basically, I feel like Reddit is more about users generating discussion and content while HN is more about users responding to a discussion or content. If that makes sense.
I use both, obviously.