I would point out these key differences.
1. A blog gives you more control. With a blog, you can target the keywords you want to target. You can edit the content after you've published it. You can time when posts go out. Of course, it takes a lot of effort to come up with good content, but you're in the driver's seat.
2. A forum gives you less control, but you don't have to write all the content. The attractive thing about a forum - for SEO purposes anyway - is user-generated content. This is great because you don't have to do all the heavy lifting that real writing requires. The bad thing about a forum is also user-generated content. All those things you have control over with a blog ... you kinda lose them with a forum. You could exert a lot of control over your forum if you want, but then folks will stop contributing and you won't have a forum. Also, all that heavy lifting you save by not having to create the content may get replaced by the necessity of moderating the content.
3. You can build a blog steadily over time. With a blog, you can go from "Hello World!" to a good following just by disciplined, regular, quality publishing over time. You can start small and build.
4. Nobody likes an empty forum. Unlike a blog, a forum needs to go live with content already in it and conversations already happening. If you're going to launch a forum, I suggest getting some dedicated customers involved in a pre-launch. Seed the forum with a number of conversations before launching it to your full customer base. Otherwise, most people will get stage fright and you will have wasted your launch.