Although I like both, I already had a blog in the past and I did not have much success, I would like to have a forum because I think you can form more vibrant communities and in general I like forums more, but I think they are also very difficult to handle, I vote for forums.
I run both as I have two forums and a blog that I run and help run. As much as I tend to spend more time on forums, it is nice to have a change when it comes to working on a blog as well.
Choosing a favourite between both is difficult for me as I like them both equally.
Blog is easier and running a forum is more fun. I vote forum because dealing with real people is more enjoyable to me than writing articles to appease search engine robots. It's more fun to run a forum party than a blog library.