Well it depends on what it is really..
And if you can even have es as plural.
I would say if you have the $$ get both because your end user could just get the plural - sell them as one, or if theyre only interested in one dont push the other on them.
On es vs. s, the right spelling is the best way to go. Singular is usually better but not always. Again, varies on what the domain is. Singular usually gets more type-in.
It all hinges on that domain. In most cases, you're kind of closing the front door but leaving the back door wide open. Other times not.
For example, domainmagazine.com is far superior to domainmagazines.com so not much harm done if you don't register domainmagazines.com. In contrast, freebooks.com is better than freebook.com but you would not want to choose one over the other as they are both awesome.