From the pictures in the fstlane's posts, it seems to me the 964 sticks out slightly more in the free airflow below the car. Also I like that they have a smooth surface, that won't interrupt the airflow. The 997 seems to be made to click on the 997 control arm, since that does work with the Toyobaru arm, the install looks kind of messy to me.
Check out how clean the 964 install looks in comparison:
http://www.ft86club.com/forums/showp...3&postcount=78
http://www.ft86club.com/forums/showp...4&postcount=80
Vs.