blogs earn money earlier then forums. forums take really long time to get established, plus lot of effort.
Forum if you can get the signups. Blog if you can get a lot of quality content. Both together if you can manage it would be even better.
I would do the opposite, i recently started my forum and when i get traffic and understand which bits of money making my members like i will know what are the best and most popular blogs to write. Also i prefer forums as it is near unlimited the amount of topics where as a blog is whatever you know about or want to write about.