Neck pickup, definatelly. I play 90% of my parts with fingers, but i use my thumb sometimes as well. I guess if you slap much placing your thumb on neck would save your time, but i can't place it anywhere else than the pickup - habits...
Other thing is if you place it on pickup, it makes your lowest string plucking faster a little, because your fingers has a point to stop on - not like when placing your thumb on neck.