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It depends on the kind of content you are going to have there. As you mentioned, if you expect good quality inks and a lot juice there, I'd suggest doing it on a sub-directory. However, if you are going to be creating many pages that could be considered 'thin' (little/no content pages, no backlinks at all, no images, etc), I'd say go with the sub-domain so as to have a buffer of its possible repercussions on the main website.