"forum" is the preferred one, but the difference is quite minimal. If you have "forums" you will most likely still rank first for when they search "xxforum" as well. But the smaller the domain, the better, and that is the only difference I can think of.
I think you need to make it sound grammatically correct as the main thing. It's difficult to know without any context, but cake-forum sounds better than cake-forums (as an example) because it's a forum, for cake discussion. Plural forums suggests more than one forum, which is great, if that's actually the case.