Its depends on the name. And the individual preference.
For example herbs or herb, I think most would go with herbs.
Car or cars? It would be Car singular in this case.
vacation or vacations? I think it would be vacation another singular
Recipe or Recipes? I think this could go either way
Assuming both variants are available then it depends on the use.
For non-developed domains you're probably better going with search stats or other measures.
For developed domains you're better off going with the one that is most relevant to the content of the site. I tend to favor how the word will be used within the site as well.
I think you could also ask:
If you registered the singular and plural, which would you develop and which would you redirect to the developed site?