Personally, I prefer the singular. I consider a "forum" to be the whole place. Some people consider sub-sections to each be a forum, making the place "forums". Different POV, probably both valid.
If you have the singular, there is no real need to also register the plural, but that's for you to decide.
The best argument would be to prevent a competing forum to be established on the plural name.
Now, they both seem registered. Maybe you have both. I would go with the singular and redirect the plural to the singular. That's only my own opinion.
PS: DNforums is for sale on afternic. Not owned by DNforum, not redirecting to it.