If you add a robots <meta> tag to a page set to "noindex,nofollow", it will remove that page from the index and stop any flow of PageRank and other link factors. If you block a page in your robots.txt file that is already indexed, it will prevent Googlebot from crawling that page so it will also prevent Googlebot from ever seeing the robots <meta> tag. An indexed page that is blocked via robots.txt will likely fall out of the index over time, but it can take a good while since Google tends to retain pages. And as long as the page remains indexed, it will flow PageRank. So, if your goal is to block the flow of PageRank etc., it would be best to use the robots <meta> tag and NOT block the pages with robots.txt.