I don't use a pin because I don't want to have to enter one every single time I want to quickly check something on my phone. Those extra few seconds would be a many-times-a-day annoyance, and I try to keep my day annoyance-free.

Since a fingerprint scan is faster, it is security I would probably use. (And the hyperventilation about hypothetical 5th amendment issues doesn't bother me one iota.)