The commentators on hacker news seem to be fewer in number and the focus and readership seems to be more about web technologies and startups.
I would be lying if I said that was not somewhat true. Hacker news is based around the startup community. However, they talk about lots of interesting technologies, some offtopic stuff and of course some startup bs. The comments are very insightful due to the experience of the commenters there. I generally look through the comments before I even open the article (if I actually open it). They also do a better job at keeping the trolls away I think than most sites. Just for example, the founders of Dropbox got their start on hacker news and still post there. There's a lot of creative and cool ideas talked about in general though on there that anyone that considers themselves a geek would like.
I do get irked by some of the code snobbier that goes on there, but you get that anywhere at any time. I just ignore those kind of discussions like anyone that isn't looking for a battle does. However, no site is perfect and like slashdot, I consider it just one of a few sites worth reading over for tech news. Lots of the articles here tend to be mirrored in discussions there (though not quite so much on political things).
As far as I'm concerned Hacker News has a higher SNR.
As mentioned, it does have some stuff worth skipping over, but Slashdot tends to suffer from the SNR effect as well with some of the articles and the lack of concrete, insightful content in comments at times as well. I just read both and accept that every site has its faults.