Grease is a feeble anti-sieze. Copper slip (one of the trade names for which is Copaslip) is copper granules in a suspension of grease, and it is the copper that inhibits the galling effect that occurs between metals.
Grease without copper can act as an anti-sieze only if you keep a thin layer of grease between the two metals to keep them apart - which in the case of a tight-fitting seat post is unlikely to be the case.
As I understand it, there isn't much difference between Ti Prep and other forms of copper slip, it is just a brand name targeting ti owners.