GalCiv 2 might have a more advanced AI, but Civ 4 is the more challenging game for me because of the game rules. I can beat Suicidal in GC2 fairly regularly, but I can't beat the highest level of Civ4, even though the bonuses there are smaller. In GC2, there are some rather simple things you can do to get very big advantages, such as tech trading, surprise attacks, fast ships, big shifts in spending, and matching weapons to defenses, and the AI doesn't do these things well, or at all. But Civ4 has a better balanced set of rules. Even a super-ingenious move doesn't result in a big advantage, and every choice has pros and cons.
The GG2 AI is good, but it is hampered by the fact that it is in a very "tricky" game. Some choices are clearly more powerful than others, and others are very wrong. The AI sometimes doesn't use the really powerful moves, and sometimes it falls into the choices that are very wrong by designing a ship that has high defenses but little or no attack, building soil enhancement when there are empty tiles on a planet already, and things like that. Civ4 has fewer choices like that, if you put the "wrong" promotion on a unit, it's no worse than an unpromoted unit, no city improvements are useless, if you lose a race for a wonder, you get some money back, etc.
It's hard to tell which AI is better when they're playing two different games. I think Civ4 is balanced much better than GC2, and this makes its AI look better, and put up a stronger fight.
GalCiv2 Forums
Try the latest 1/8 build.