I think cases are designed with the idea of a flow through design to channel air though the case, not circulate it or form vacuums.
I have not done testing but I believe air blowing onto the heatsink will be more effective since air particles are being forced onto the metal to absorb heat rather than the suction effect and air displacing itself.
In a computer case you don't want air pockets that circulate air.. you want a steady flow of air to pass through the case. Intake on the bottom front of the case for cold air induction
and then a blowhole on the upper area of the case for hot air exhaust (hot air rises).
If you put fans on top of eachother blowing in/out, then it acts as a cycle that doesn't do much good in passing air over all the components.