simple decision for me, all things being equal (same amount of outbounds on both pages, same code, etc,) with the exception of the PR8 being off topic and PR6 being on topic... I'd choose the on topic hands down. Better quality traffic for you since it's "related" rather than wasted traffic.
Why refer people from a website dedicated to apples to one about oranges when you can get more effective conversions etc. from referrals from a site about oranges?
Think about the surfers, they would appreciate an "on topic" link more.