Study on Miles Redemption
Generally speaking value-oriented airlines have much better award seat availability than the full-fare carriers. In a study done by Switchfly, frequent flyer programs like topbonus (air berlin, NIKI) and Rapid Rewards (Southwest Airlines) showed excellent seat availability for frequent flyer program members.

The study also looked at trends and indicates that member rewards are getting better across the board. Two full fare programs that were singled out for their improved seat availability were Executive Club (British Airways) and United Mileage Plus.

Article in the Chicago Tribune:
http://articles.chicagotribune.com/2012-05-31/business/ct-biz-0601-airlines-notebook-20120531_1_frequent-flier-programs-frequent-fliers-ceo-tom-horton