Personally Ive had some bad experience with Xigmatech fans. I need to run mine with 2 resistors for them to stop "whining".
The Corsair SP120 fans are a very good option for pushpull configs for a specific reason.
Usually fan quality is measured in CFM(airflow) and DB(sound). This doesnt really take into concideration the direction of pulled air or the air "leakage" backwards etc. The Corsair SP-fans means "Static pressure" and they are designed to push air straight, they have slightly less airflow than normal fans but they push it very linear, which is great for push/pull configs.
If you just want added airflow from somwhere, Corsair has AF (AIR-FLOW) fans too. Higher CFM at the same DB but more leakage etc