But what if the version that brings more traffic to the site is the one that people search on the least? It's better to find out which version is searched on the most and aim at improving that - especially if its the version that brings you the least traffic.

Google incorporates stemming into their algo, so pages can rank well for the plural version without actually having it on the page, or in any links to the page, and vice versa. But their stemming only goes as far as getting results from the index. When it comes to actually ranking the pages, exact matches do make a difference. So the advice about link texts is good. Personally, I would include both versions in the Title, and in the link texts pointing to the page.