goodroi - 11:23 am on Aug 18, 2011 (gmt 0)
Link juice is a generic term referring to backlinks. There are many ways to measure link juice (amount of unique ips, total volume of links, etc.). The term link juice is good for when you have to simplify the topic matter to help clients understand basic SEO. When doing research you probably want to better define the term so you can accurately measure the impact of your test.
Pagerank is a specific way to measure backlinks that was designed by Larry Page and patented for Google. Real Pagerank data is not shown to the public. We see Toolbar Pagerank (TBPR) which is a snapshot of Pagerank data taken weeks/months ago and Google has been known to manipulate it.