What is pop_model and which one should I use?

Popularity model specifies which object is request among objects in the working set (objects that were requested before) when Polygraph revisits a URL (see Robot.recurrence). Popularity model matters only if Robot.recurrence is greater than 0.

Popularity model is described at http://www.web-polygraph.org/docs/reference/pgl/types.html#type:docs/reference/pgl/types/PopModel.