chassi fans

what is more efficient way of cooling..... two fan in the back that suck the air out or two that blows the air in or one that blows and another that sucks.

2 pulling air out, in my experiences. Ideally, you would have equal incoming/outgoing airflow through the case but that's very hard to achieve. Also, you have to take into account potential deadspots where the air is not moving/leaving.