The 993 rears are also slightly narrower than the 996, allowing you to fit them under 17" wheels. I'm not sure whether the 996 rears need 18s but I believe the fronts do.
IIRC, the 996 are tougher to find and more desired as upgrades for 993 owners and Audi folks. The 996 went to monobloc construction while the 993s are two-piece calipers. (I have 993 fronts using GS-made adapters.)
This may help: