Obviously a topic to start religious wars
. I'd start by looking at what makers like Obsession or SDM do. I imagine they have done their homework. Digging around in the recesses of my memory I think the answer is 'both'. Blowing does a better job of cooling because the air is forced more directly onto the mirror but the air coming around the mirror generates turbulence in the tube. Sucking is less effective but the air flow in front of the mirror is smoother. So blow for initial cooling (until the mirror is just above ambient air temperature) and then suck to maintain the relative temperature as the air cools during the night. There is also a school of thought that you put the fan in the side of the tube just above the mirror with an air inlet opposite and draw air across the front surface.
Of course in the UK you just leave the scope inside and curse the clouds
(he says looking out the window at the rain).