If you want the best of both worlds in one spot, you can use a tool like WordTracker found here:
http://www.theadtool.com/wordtracker. This very useful tool will give you counts from most search engines, distinguish between singular and plural, and offer related
terms with their counts. It also provides an index number called KEI to give you an idea on how competitive certain terms are (mostly for organic search engine marketing).
Regardless of which tools you use, it's critical that you grab hundreds of keyword variations and put them into your ad groups. If you don't, you're probably missing out on some very low priced terms and might even be missing out on what everyone is typing
when searching. If you know the story of the insurance companies promoting auto insurance instead of car insurance, then you can certainly see the importance of including related terms.