I'm assuming that by "having your own blog" you mean a link blog on which you submit links to stories you find interesting in your area of interest (as opposed to writing your own posts) and others can read it just like they read reddit or digg. In that case, I'd say that a social news site is better because it won't stop when you're on vacation or not feeling like posting. However, for a very small domain for which there is not a big enough community, a link blog is the next best thing.
I mean both actually, not just link blogs.
You don't get as much SEO juice by using social news services. There is a little element in the HTML markup of some social news sites that essentially tells search engines to ignore the link. This means you don't get credit for that link back to your domain. In addition, having your own blog can increase the number of times search engines visit the rest of your site...again, SEO 101.
Why have your own blog if you can just post to a social news service for your domain?