I personally prefer forum. But set up redirects of anything anyone is likely to type in as well. /forums, /board, /community, /bbs etc... all redirect to /forum. I always thought of it as a forum with a collection of sub forums. I see no real difference in which directory you put your forum in so long as it can be found. All a matter of personal preference.
edit: I believe fora is the proper (latin) plural form of forum.
I use forums personally, ever since my first install and in the instructions to install a fresh vbulletin is for a folder called forums, so I used that and then just stuck with it from there.