As far as I know, the Saturn game and first PS1 game are the same. Culdcept Expansion Plus on the PS1 includes all of the maps from the All Japan Cepter's Cup at the time, but it doesn't include any new cards. The game save is interchangeable between Culdcept Expansion and Expansion Plus.
Culdcept DS is actually an updated Saturn/PS1 game (and much better). It has many refinements, such as some new cards, many more maps, lots of free downloadables, more avatars, and some much-needed gameplay improvements (such as fort bonuses - the Saturn/PS1 game has none).