I'd go for AI! The problem I have with Civ is that they create these really sort of interesting and elaborate systems, but they've clearly long surpassed the capabilities of their AI systems to the point that the only way to create the illusion of competitiveness is with giving the AI ever more massive cheat bonuses (200% faster research, 200% more resources per tile, etc type stuff).
That really ruins the 'real' game and creates this sort of meta layer where you're specifically aiming to compete against these cheat bonuses rather than actually playing the game. It'd be analogous to a really weak chess AI trying to create the facade of challenge by randomly giving itself a new pawn every 2 turns, a a knight every 5 turns, and a rook every 20.
That really ruins the 'real' game and creates this sort of meta layer where you're specifically aiming to compete against these cheat bonuses rather than actually playing the game. It'd be analogous to a really weak chess AI trying to create the facade of challenge by randomly giving itself a new pawn every 2 turns, a a knight every 5 turns, and a rook every 20.