Should I use plural or singular keywords?

Is it better to for plural or singular keywords or go for both?

For example:

great keyword
great keywords

Check your google keyword tools, and see which one gets more hits

yes, whichever gets more searches, mostly I go for the plural because it already includes the singular keyword while the singular doesnt include the plural.

Thats where I'm little confused, because if the plural form includes the singular why would I go for anything but the plural version.

Also in most cases the plural version has more exact matches anyway, but there are some cases where the singular version has way more matches then the plural one.

I'm also noticing that in the search result I get are quite different with the singular and plural version.

Is it wise to just target both?

There's no cookie cutter way to do it. No generic answer. If the singular version gets more searches and the results better suit your niche, then better go for it. Simple concept, really.

Just check which one has the highest search number on google keyword tool and use the exact match search criteria.

Also having a plural/singular keyword probably doesn't matter that much, if you are ranking for 1 you will most likely be ranking for the other as well.