A tip helpful for the decision whether to enslave, exterminate or occupy a city: When the window with the coice pops up, you can still scroll the strategic map in the background (at least this is te case with patch 1.5, not so with 1.2). Scroll until you can see the city status display. If the city is red (assuming you´ve attacked with a full-stack army), it´s best to exterminate, you´ll get rid of most of the troublemakers. Enslaving might not be enough, if the city is in the red. Enslaving can be used when te city is in the blue or yellow and I´d only occupy if the city is still green after the conquest.