Agreed there needs to be a definte cohesion between the site and the forum (ie - more than just a "forum" link in the main navigation).
I have a content site (articles) which was intended to cover the basics of the subject are and the forum was in place to discuss new or specific issues.
Although the forum isn't hugely active in the grand scheme of things, it still serves its purpose well (the site gets around 1.5 to 2k visits per day, the forum gets a handfull of new users and perhaps a dozen or so posts, although this is steadily increasing).
The forum usage is pretty low considering the overall traffic of the site, and this concerned me to begin with, but in retrospect, it is only there as a secondary resource - the articles are the primary reason for going to the site (the articles cover commonly asked questions in the subject area).
I would say this is the case with most content sites - people go for the content, not the discussions so the forum will always be secondary.
In terms of SEO, I would say having a content site with forum is infinitely easier to develop - people are more likely to link to it to begin with.
You could write 40 articles on a subject area and post them as content in your main site and with navigation, etc it would give the impression of a reasonable sized site.
Take the same 40 articles and make them forum posts, not articles, then people arrive at your site and see a forum with one user and only 40 posts.
Although in both cases the same information has been posted, hosting it within the forum in the 2nd example makes it seem so much less.
When you design a content site, the site itself is a container and the amount of information you have within it fits perfectly. However, a discussion forum is an infinitely larger container therefore the information appears to be proportionally smaller.
What works better in the long term? Probably down to the industry - are your visitors going for information or for a discussion? And what's better - that they find the information or have to ask?