Scihub solves an entirely different problem.

I wouldn’t call Unpaywall a copycat of sci-hub. Unpaywall helps finding links to the same article hosted elsewhere, e.g. on a preprint server or on the author’s page. It is more similar to specific features of Google Scholar or Semantic Scholar, with the small additional convenience of appearing directly on the article web page, instead of requiring a search.

I'd probably use Scihub for a quick download for a paper I'm interested in reading. And then if I wanted to share it publicly or with coworkers I might use Unpaywall.

Just yesterday Unpaywall found a publicly available copy of a 1980s article I was looking for and which Sci-hub didn't have.

I don’t know why it should be a question of which one to use. Use both because both do different things.

Unpaywall has very little to do with scihub: This isn't about giving access to works that aren't publicly available, but instead about giving links to existing free uploads of the material by the author (i.e. on arxiv or his/her personal website).

Scihub and unpaywall are entirely different endeavours.