The most appropriate Jesmonite product to create a smooth polished surface for a master/prototype would be Jesmonite AC100.
Once fully cured the AC100 surface can be finished with sandpaper prior to moulding. For finishing AC100 surfaces smoothly, use a super fine sandpaper with 360- to 600-grit. Once the AC100 has been sanded you could then buff the surface with a white cotton pad to achieve a polished effect.
You could achieve a similar polished surface with AC300, however the finished/cured AC300 product wouldn’t achieve the same strength and durability which you would gain from AC100.