Go with a blog if you are writing about things you have enough idea about. If you are not the wisest person around in the area you want to start your site with then go with a forum as you will have enough input from people. Blogs are considered more valuable/resourceful as contents usually are always very relevant.
Forums take ridiculous amounts of dedication. Also there are so many now unless it's UBER niche; good luck. Just remember you have to be the most enthusiastic about it LONG-TERM or no one else will. If you do start a blog by all means come chill at the blog forum ---> Still a work in progress
I suggest a blog, they are quicker to get going and still easy to work with, i have one forum, quite a tight niche and its hard to get it moving, its got members and ive posted but its not easy.