Here is a free book that compares a lot of different self steering systems written by Peter Forthmann who is the manufacturer of Windpilots. He is biased but he makes a very good argument for his system in the book. The free book is available here:
http://www.windpilot.de/Grafiken/pdf/bookeng.pdf
Careful pressing the link as it actually downloads the free book.
Peter Forthmann's website is here:
http://www.windpilot.de/