One thing in question with nearly all domain names is, plural, singular or both. Domain Names that have the potential of being plural or singular should always be owned by the same person when possible.

Do site owners always own both? No, but again it is always best to own both if you can.

Also, in many cases, one may think the plural or singular versions has more value as one will likely be a "more common term", be more clear with it’s message and get the most traffic. I would agree with this!

Plural terms often make more sense to be the "Main Domain" used for a site when it comes to generic terms.

Example:

Tool.com or Tools.com

Tools.com to me is a much better domain to be used for a tool brand or tool store. It’s not like the site would offer One Tool, so that is my main reason why I like the plural. I would also value Tools.com higher then Tool.com .

In some cases, the singular version is more common and holds a higher value to it’s wide range of use.

Example:

Football.com or Footballs

Football.com can be used for anything football sports related (in the US) where as Footballs to me is the Ball used in Football. Footballs.com would best offer footballs themself. In this case, the singular version of the domain name would hold more value.

With branded terms and just other basic terms is likely when most questions come into play. If you think somebody may use direct navigation for either version you should own both if you can.

Here is an example of two recently reported sold domains (highlighted). One plural, one singular.

You will notice the plural domain sold for $4,000 more then the singular version. Is this always the case? No, but as I explained above, the difference of one letter can really make a big difference in the value of a domain.

In this case, for a business the Plural term makes the most sense to use as it’s main sites domain, but the singular version would be considered a more likely "search term" for users on the net looking to rent one. Owning both domains covers your site for the typo chance  if advertised with a plural or singular versoin as well if somebody makes the term plural or leaves it singular when typing the domain into a browser (direct navigation).

Bottom line: Own both plural and singular and it’s likely you will pay More for one or the other.