There isn't a "right approach" as you cannot guess what actually will happen in a specific seismic event.

Rp is just a factor that counts ductility effect. Basically, in building seismic codes, a high value is associated with the ability to develop (in specific places, by Designer choice) some plastic hinges. Probably being to the limits of elastic calculation with Rp=12 means for the system to survive with non-elastic strain. Which may be acceptable in some circumstances or not acceptable in others. Just my interpretation of that value of Rp=12, of course.

B31E is a prudent approach and anyway it considers much more things than seismic accelerations and Rp values.