This is always a discussion I see that really does not matter all that much. Do not write content with keyword density in mind. Write it on topic and for the users and it should com naturally. If you focus too much on pushing your keyword in teh copy then it will look spammy and may be hard for readers.

As for length, I say the more the better. You do not want a page with nothing on it but you also do not want pages and pages of absolute nothing.  500-1000 words i think is a good range

The way I have always looked at this is to add the 's'

That way you have the singular AND plural keyword covered.

The singular is of course in the url as well and that s just makes your site more optimized for the plural too.

Then to really differentiate which one you want to rank for, just change some the other on page factors and link building