Search engines rank web pages, not websites. The PR of the page where you place your backlinks is the only PR that matters (Websites aren’t assigned PR only pages have PR). With that principle in mind, the relevance of a website topic is not important, however, the relevance of a webpage is paramount.
Much of the time we can make a page that we place our backlinks on relevant by including our targeted keyword as the anchor text. We can also influence the relevance of the page where we place our links by creating additional links to that page using our targeted keyword.
If you have the opportunity to get a link on a page that has PR 7 or 8 then you should definitely go for it. Which of those two are better can vary based on many different factors, including the total number of outbound links and how relevant you can make the backlink to your keyword.
Aside from the SEO benefit, of prime importance is the potential traffic. You are more likely to get direct link traffic from a relevant web page on a website that gets targeted traffic relevant to your targeted keyword. Without having more specific information on each link opportunity it would be hard to say for certain which would be better.