The S62 Z8 was the original, 400hp 6-speed supercar.
The 4.8L model you're looking at is the one-year special Alpina edition. Automatic tranny, 20" Alpina wheels, and other little touches. It's sort of a small special edition to say goodbye to the Z8 production. Likely hold collector status 50 years from now.
They were never produced at the same time, the Alpina model replaced the original S62 variant.