Private Advertisers. You'll make little with contextual or anything. Forums are primarily for discussion so it is difficult to monetize content such as that. CPM would work as well.
Adsense does seem to do better inline on forums with the right amount of blending but if its a niche forum then there is probably a better opportunity going with an affiliate program.
You need a TONE of people to cash in on adsense also typically people who click on adsense ARE NOT webmasters. I believe DP does so well mainly from the clicks of visitors who come through search engines and NOT from it's members. I've never clicked a DP ad... I could be wrong but from this and my experience on my forum adsense is NOT the way to go. I made it rev share so I'm stuck with it.
If you are driving some traffic...you can get some affiliate program owners to advertise on a monthly basis. I might be interested if you are seeing some decent traffic...let me know.
It's a webmaster forum -- it says so in the first post. I'd go for Private Advertisers. I never had luck monetizing any forum with Adsense. But it does work with DP. You might want to try it out one by one and see what works for you.