I'm planning to create my own cooling pad for my MacBook Pro (2011).
Which is the better airflow direction - is it preferred to suck air away from the MacBook, or blow it towards the Mac?
Assuming the case underside of the laptop is a smooth, nonperforated plane, you should ideally force air onto the center of the plane and have a second plane mounted beneath it so that the incoming air is forced to pass over the whole surface to escape at the sides. Inverting the flow gives greater inequalities in distribution due to the computer not being circular, in/outflow having an area and possible different levels of obstruction in different directions.
The most important thing would be to make sure the airflow is approximately the same everywhere on the surface you want to cool, and to make sure the cooled surface is as large as possible.