1st question: So wouldn't you want a combination of both then?
Hypothetically, if you created each of these backlinks to for three exact same websites at the same time:
A) 100 PR 5 dofollow links
B) 50 PR 5 dofollow links/50 PR 5 no follow links (link diversity)
C) 100 PR 5 nofollow links
2nd question: Which would rank the fastest if each went at the same link velocity? In other words, which method is more effective?