I would recommend using the noindex meta tag over blocking pages with the robots.txt file. Either will achieve the same result, but the noindex tag is a superior solution for several reasons:
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the noindex tag will allow a crawler to see the rest of the page and for your PR to continue it's natural flow throughout your site.
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there are times when a robots.txt file is deleted or modified in error, and then you have a big headache on your hands. Even if a meta tag is erased, it would just affect a single page so the damage is a lot more manageable.