I see virtually no difference with the plurals. I will register a pluralized domain in a heartbeat. Honestly I believe that Google's algorithm accepts a singular/pluralized version of a word as virtually the exact same thing in the domain, much the same way it does while performing a search.
Hyphens I can't speak from experience, because i've avoided them like the plague from day one. However, I still see a whole lot of hyphenated domains ranking on reasonably competitive terms while doing keyword research. So I'd have to assume if you have an exact match for your targetted term with a hyphen in it, you're still doing ok.
My big favorite for SEO is the reverse matched 2 word term on a .com. Well, second to the exact match at least...
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