There is no answer to that. Both can be equally profitable depending on what you do to earn money. Forums are much more work and take a larger upfront investment to get membership up. A blog is easier to start and can be done on any type of budget. Either way you go, it will take time and lots of work to build up traffic and promote the sites. If you look at DP, Shawn is making good money. For every money making forum, there is most likely thousands that fail to make any real money. On the otehr side, you see people like Shoemoney who has great profits with his blog. For each one like that, there are most likely thousands that fail to make any real money. Go with the one you know the most about and have the most time to manage.
I've looked at this issue before and I see it this way, with a blog site your limited to your index, and archive pages as far as search engine indexing is concerned. You also need to keep on blogging to support content to search engines and your loyal readers, This can be laborious and you risk the possibility of a low income (say if there was a poor PPC or little response to paid text ads in a given month!) On the other hand if you were to develop a forum site not only do you have more site freedom ( sub-domains, macro dir etc) your loyal traffic provide your site with the content. This free content can free up your time to search for links. The virtual equity value will always be greater with a forum site than with a blog. Think of it as your bonus supa, some- thing to look forward to when selling such a site! I have a video/forum site which I'm planing to release online in a few months time, so I'll let you know how it turns out! PS:a little SEO webmaster trick up my sleeve that you or anyone else for that matter can do to promote traffic to your new sites. Aquire expired traffic domains and bank together as a sort of 'traffic pipe line' and as such channel this flow to any new forum or blog to build up your membership base and Alexa rank. It's sad for me to see good looking websites in ideal keyword spots fail because of the failure to act upon such simple solutions as mentioned above!