Actually, I've played both games extensively before. I bought both right after they came out for the PC (bought a Mac in 2000). I've since moved a few times and lost my Alien Crossfire PC CD and managed to scratch up my SMAC CD. The game is amazing. The diplomacy is incredibly realistic. It's just like the UN in my opinion. I am playing a game right now and I have the largest military, and I'm focusing research/economy/etc on my military. The result: most of the world resents me because of my large forces. They let me know this directly ("you may fool others by your quest for "knowledge," but I know it's just a face for military spending," or something). It's very interesting. It blows away the Civ3 AI. It is really a completely different game than Civ3 (fairly similar to Civ2, which I bought when that came out also). In AC, we can play a civilization that knows basic modern aspects of government/economy/etc, and there is, therefore, much greater flexibility in choosing an economic structure, etc. I would really like to see an Alpha Centauri 3, maybe with an interface more like Civ3 (brighter, more visible, better advisors). The game has so much potential. It's a shame they just gave up on it. Overall, it blows Civ3 out of the water.
-Jim