Sounds and looks similar, but different game focus. Imperium is more detailed on the strategic/tactical aspects of the game (i.e., tactical battle games + detailed troop system + more detailed ground scale), and less detailed on the political side of the game (e.g., Pax has a 1 year political cycle, whereas Imperium's cycle is 3 year and probably more abstract).
Will be interesting to see it when it comes out. A sticking point for the game will be whether the AI will be any good at negotiation = traditionally a weak point in artificial intelligence (due to cold, scientific reasons).
