I think the hardest time to pick between the two is when it's a sub domain or directory.

Usually, whether it is "forum" or "forums" completely depends on the word before it. If it is an adjective, then the word is usually plural. If the preceeding word is a noun, then it's usually singular. For example. "runningforums" vs "truckforum"

Although "runningforum" and "truckforums" also works and sounds decent, the adjective / noun thing is something I have really noticed in the past.

As far as breaking it down after the url such as "www.domain.tld/forum" and www.domain.tld/forums" I think it really is based on personal preference. For me, I like to use "forums" just because it sounds bigger, and "better" :-D

may be sub-domain names can do the job too
use sub-domain then you do not need to buy extra domain names

I checked google keyword q="forum" it does not show many "forums" for the top index rank....

Also for google keyword q="forums" most of the top list are using "sub-domain" or "folder" only

personal.... I prefer "forum" .... but I will put "forums" or "talk" such keywords at title head meta

Originally Posted by khalid

buy both domains, use "forums" and forward "forum" to it.

This is what I do, then use the one that sounds the best.

i like forum(s) better but trying to get away from using that word anymore

Forums is better it makes it sound a bit bigger that Forum