I've seen similar things happen...what I try to do is one of two things...
1. If I can, I come up with a creative(and natural) way to work both into the title and then work both variations naturally into the content. I don't tend to repeat a string of singulars and plurals in the url. I just pick whichever is the better traffic keyword.
2. If I cant come up with a way to work the singular and plural into the title, I pick whichever is the better of the two options(let's say the singular) and use that one in the title. Then I'll make sure to work the secondary term(the plural) into the content of the page. I'll try to incorporate it into an
or
tag too if it makes sense.
I've seen people saying that Google treated the singular and the plural versions of search terms the same...but I've seen evidence to the contrary in my experience.