This is one of those threads that you will only really get feedback from people who can recommend 1 system...the one they own. I would venture to guess that there are VERY few people who have multiple systems and even fewer who could tell you the actual differences.
So essentially, you have to decide for yourself what you are after:
How do the arms adjust? (camber or caster)
I will say one thing I like about the Holz arms I had (not long travel though) is that the shock is moved out on the trailing arms (less of an "L" shape) This reduces the load on the radius rods and also makes it a lot easier to get the clutch cover off. i.e. no need to remove the shock. I think I read one here at one time that the Cognito arms do the same thing although I unfortunately have no first hand experience with them.