i think the main advantage that reddit has over HN is the combination of breadth and specificity. there aren't any categories or labels in HN and, as has already been discussed, people generally only see the front page. this leads to the "social echo chamber" effect that people have discussed elsewhere, meaning a lot of people read/respond to the same ideas over and over leading to an incestuous breeding ground for thought/commentary.

i prefer to create my own multireddits for content technical content i'm interested in discussing. one for languages, open source, web, mobile, or computer science theory. these would be impossible to find on HN but on reddit they often attract more serious comments (not perfect).

also, did you ask this same question on reddit? i'm wary of the implicit selection bias in asking for a comparison from HN users and not reddit. the users on HN who take the time to read and respond to something like this are more than likely to prefer HN to reddit.