When it comes to decide the keywords, I met a dilemma. which one to choose, the singular form keyword or the plural ones ? Of course, it's better to put both forms of the keyword in your website if possible, but if not, which one to choose ? What's the respective advantages of the two forms ?
Check out what your visitors are searching for through google analytics. It will give you a full list of all keywords and which words have brought in the most traffic to your website and how long they viewed your page for.
It depends. If the word is often used in its singular form then use it that. If it is used mostly in its plural form then use the keyword in its plural form.