I can agree with Nick on this one - there's just something about the actual look of the directory that is important to me.
To an English Major it would make a world of difference, they'd be pondering the conundrum for hours between asking "Would you like fries with that?"
I've used both, but usually use "forum" unless I'm setting up a new forum on the same domain. Then I set it up under "forums" and usually don't bother to change it.
I just use my root. But if I had to use one, I would use /forum as there is only one board and multiple boards as /forums suggests.