I suggest you register both the domains and then park the other domain on top of the main domain. This way you will even get typo error traffic. Some people will type forum and some will type forums. Also, these days, the biggest problem is that people register the other variant of the domain and then try to take advantage of it. For e.g. you register petforum.com. So someone will register petforums.com. Obviously it will only happen if your site get lot of popularity. But still there's no point in taking any chance.
I prefer, forums. See DP it uses forums. Use forums if you will create variety of area/topics on your forum site. That is my opinion.
forums sounds better and is searched a lot more times..however it is advisable to get both if possible..
Maybe do both forum and forums.... I like "FORUM" best Singular is the way to go if you have to choose one...
Some people consider a forum to be the entire site with each different section inside it considered a "subforum" but others feel that the site considers multiple forums which are the different sections inside it. As others have said, get both versions if you can and redirect one to the other. I haven't really decided on a preference though.