I have the following question: Which link is better (passes more authority / increases pagerank / etc.), a PR6 link with about 100 links or a PR4 backlink with about 15 links on the page?
PR isn't the only indication of authority and so the PR might not make much difference. And even though it's better to have your link coming from a page with less links, there's no rule that says more links will definately hinder you.
I cannot tell you the answer with only little information about those 2 links. There are more many factors use to judge link value and not solely on PR.
Assuming they both offer relevant content to your site, I would personally go with the PR6. From personal experience 100 OBL isn't even that many.