gRegorIt depends on the type of post and context, what you intend consumers to see. In most posts, if you want a tag/category name to be the result, you want p-category. Some indieweb cases like people tagging use u-category, though: https://indieweb.org/person-tag#How_to
ulhar4409although, if the link points to another website entirely with the intent of being a submission for *that* site, would that be a good usecase for u-category?
gRegorWorth checking out IndieNews an indiewebxyz, they didn't use p-category quite like that, though they could. A syndication link like IndieNews uses works to establish the intent to submit the post
ulhar4409there is a sort of "remarks in tags" pattern from tumblr/cohost that i would be wary of here, which could probably be sidestepped with some kind of "Please don't p-category your tags if you're not using them as categories" policy