Technically, it's the URL that has the link juice, and not the content. Moving it, even with a 301 redirect, is likely to cause "some" loss of link juice. Since you have the option, my preference would be keep the article on the domain where it has always been (cool URIs don't change, and all that).
Then link judiciously from that article to your other site - just once would be best - and have few or no other outbound links on that article page. That approach should pass the maximum link juice.
Since the site containing the article is in the same niche as your current active site, for maximum link benefit, I'd recommend that you pay some attention to keeping the link profiles of your current site independent of the links to the article site.
This is particularly so if they share hosting. In my experience, independent link profiles are crucial in situations like this. If the sites are not yet associated with each other, I myself would make sure that the article site is on a different server than the destination site... even move it before linking the two.