Which is better for Link Building - Forum Posting or Blog Commenting

It's really not about forum posting or blog commenting. I can say that both techniques are good provided that you are doing it in an organic way. I have seen many forum posters or blog posters who just spam it. The main idea behind this is to being a part of that conversation and it doesn't mean that you just sign up and say "thanks" to a post.
If your blog comment is worthwhile, there is a possibility that the blog-webmaster or owner will appreciate you and will promote your site. And the same thing is with forums. You need to help others trying to learn things. If you can share your knowledge, they will appreciate it and will promote for you.

So in my opinion, both are good unless if you are doing it in a good organic way and not spamming!