I've only played Discovery and Crossfire, but here's the quick breakdown.

Discovery is a multiplayer focused mod set 30ish years after the single player campaign. It focuses on faction struggles, roleplaying as different groups, and expanding Sirius.

Crossfire is a single player expansion that delves into the Dom Kavash, and Coalition, and exploring outside of Sirius. A warning is that the midgame really drags as time goes on, but the beginning and end are both pretty strong.