That conversation can get a little sticky around here
http://www.treebuzz.com/forum/threads/current-news-regarding-the-saka-device.33678/
Personal opinion: The HAAS took the idea of a knee ascender and made great improvements on it while getting rid of some of the challenges. Adding a stiff section of material above the foot to help with tending, among other benefits. The SAKA came along later and improved the knee ascender idea even further by putting additional thought into the human/mechanical interaction adding stability and ease of use and a shorter/stronger tending cord. There are new innovations coming out on the next version of the SAKA as I type.
Many climbers on here use the HAAS and have no interest in changing, many also use the SAKA and love it too.
I started with a SAKA so I have not found an incentive to switch to a HAAS. either one will get you up a tree without hip thrusting.