I would guess the difference between the two brackets is the amount that the lower control arm is moved down. This affects the instant center created by the intersection of the upper and lower control arms. It's all about the relationship between the center of gravity of the car and the instant center formed by the control arms.I would suggest reading the following, they helped me to really understand rear suspension geometry:
http://www.geocities.com/kdslaby/Launching_A_Drag_Car.html
http://www.geocities.com/kdslaby/Baseline_Suspension.html
http://www.geocities.com/kdslaby/Pinion_Angle.html
This guy seems to really know his stuff. He also sells an upper control arm set-up that moves the upper control arm mounting point up, thus changing the instant center:
http://www.baselinesuspensions.com/
This stuff got me thinking, so maybe it will get you thinking about it too. Hope it helps!