It really depends on several things.
The airflow patterns in your case.
The heatsink design itself.
The metric used to determine performance.
Number three is a bit touchy. Many mainboard in socket sensors will indicate a lower temperature when the fan is blowing on the sink due to secondary cooling! The core could actually be hotter.
You could always set up one way and overclock until you reach the limit of stability and back off a little. Once you find it running stable, try flipping the fan around. If it crashes without changing anything you know the way you had it before was better and vice versa.
Without actually monitoring the on die thermal diode directly, there is no proven metric to show which way is superior in your situation. Well, except for the time consuming method I just outlined!
Cheers!