Feel up for to simple experiment? If your HSF fan blows into the HS, put the stronger one on the rear opening, blowing in and the weaker one on the fron opening, blowing out. Run some stress test until the CPU temp stops rising. Record that CPU temp. Now, reverse them, with the stronger blowing in the front and the weaker blowing out the rear. Run the same stressing test and record the CPU temp. See which way is cooler. If your HSF fan draws air out of the HS, reverse the test order. By stronger and weaker fans, I mean a Sunon 42.5cfm or Panaflo 47cfm as the stronger and a Sunon 36cfm as the weaker. That will balance out considering the PSU fan as a secondary exhaust fan. depending upon your PC room temp and how high you intend to OC, you might be better served modding both openings for 92x32mm Sanyo Denki 55cfm fans.
Hoot