Having a fan drawing air into the enclosure creates a rather localised fast air-flow that would increase the probability of drawing dust, dirt and chips into the ensure at high speed, and depositing this onto the PCBs/electronics. Drawing air out of the enclosure (exhaust) allows the sourced air to be collated from the various air inlets at lower pressure differential and is therefore less likely to draw detritus into the enclosure.

Chosing an exhaust solution - the logical positioning is on the top of the enclosure to remove the hot air at the top of the enclosure.

Regardless, the controller IP is poor on these and they need protection within an external enclosure - though I don't practice this myself.