Best use of a second case fan for good airflow?

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Honorable
Jul 21, 2013
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My motherboard supports 2 case fans. One of these will be the pre-installed rear exhaust, but I was wondering where I should put the other 120mm fan I have to get the most efficient airflow within the case.

I'm using a NZXT Source 210 case: http://i.imgur.com/0IbLpNP.jpg

I read that using the front intakes in addition to the rear exhaust would provide the best airflow. The question I have is: which of the two front intakes should I put the fan in? The top or the bottom? http://i.imgur.com/tvbLPPT.jpg

And if it makes any difference, my hard drive will have to be in the middle or lower bay slots due to my GPU's size.