the bottom spacer design will slightly increase anti-squat as it pushes the subframe (and rear instant center) higher relative to the center of gravity of the car. The top/bottom spacer design maintains the stock anti-squat %. The difference can be noticeable, the increased anti-squat will make the rear end feel "stiffer", but otherwise they perform the same function in terms of eliminating slack in the bushings which causes wheel hop.