If you can, if you have the time, the ideal approach would be to do both - write at your own blog, and participate with others in forums, guest blog posts, and comments.
Use your blog to post things that show your own unique voice, and the things that might interest your visitors. Be involved in the community at those other sites to learn about new things, to share ideas and experiences, and to interact with others.
A blog can be a more solitary endeavor, or at least a more personal one. Being active in a forum means that you can engage others, and be engaged.
If you write something at your blog, you really don't want to go to a forum (any forum), and just leave a link to your blog post - that would probably be seen as a link drop, solely aimed at bringing you traffic without really engaging people in conversation. And it's the conversation that is the strength of participating in a forum - giving people a chance to share their perspectives on a topic or concept or opinion, so that you can see a subject from more than just your perspective.
Likewise, you wouldn't just want to make a blog post at your blog that is just a link to a forum post - you would want to describe it, to tell your readers your thoughts and opinions on it, and to provide something interesting on your own site that gets them to come back to your blog.
Posting something twice - here and at a blog, it isn't something that I would do, and not something that I would recommend. Yes, there might be an issue with a duplication filter, but beyond that, it's impersonal - you aren't writing for the audiences at each and taking advantage of the strengths of either type of communication.
You write that"
(Here in South Africa we are on the other, not so happening, side of the Digital Divide. People here ask me what a blog is never mind what a good feed is.
It sounds like there's a good opportunity to write about blogging, to write about feeds, to educate and illustrate, and draw a big audience locally, by focusing upon other local bloggers and build a community that helps to lift up South Africa. It's not necessarily the audience that you want to reach at Cre8asite or SEOmoz, but it's one that may give you recognition and opportunities where you live.