In Chieftan the computer is downed to 0.25 of its original strength\defense point value for every unit.
for warlord it is downed to half .
prince is 0.75.
King is 1 (Fair game).
And Emperor is 1.25 (computer has the advantage)
That's why I prefer King mode.
But I could be inaccurate, but it is something close to that.
This also has something to do with the scoring; that's why it's better IMO to judge the score by the percentage you get. You can get the same score in cheif and emperor, but the percentage will be higher in emperor obviously. That means, even though the cheif game and the emperor gae have the same score, the emperor game was played better.