Visually, there is no real difference other than the rear wheels and even that is tought to see. You could only get Z51 in the coupe. The ride quality is certainly different and feels much firmer than the base FE1 suspension.
Z51 came with stiffer springs and shocks, larger sway bars, HD lower control arm bushings, quicker steering, additional stiffening braces around the front crossmember, oil cooler, PS cooler, auxilary cooling fan, HD radiator, and was available with either the manual 4+3 or the A4 automatic.