I also feel like the HN community is better an following the guidelines than Reddit. For example, I feel more HN'ers treat the downvote arrows as "does not add to discussion", while on Reddit the majority treat it as a "disagree" button. Maybe this is because you have to earn the ability to downvote on HN? Who knows, but I enjoy this community more for actually discussing things.
I'm fine with "well, actually..." if what follows is a well reasoned argument. On /r/math I've actually had someone reply that they were a millionaire, and thus I had to be wrong, followed by PM'ing me a photo of their bank statement.
More seriously, it probably depends on the tone you take. In all the times I've posted here, I've tried to be respectful and I've almost always received respect in turn, even when people didn't agree with me.
Blast it, I've just undermined my own comment, I meant to say: Well, actually ...
On a serious note, both of them seem pretty similar to me.