It was in the past that we do use past record to predict the future, but this season proved everyone wrong that past history doesn't guarantee future results. I see Chelsea as the favorite in this match, because City have lost form and are unpredictable when playing with big clubs. Imagine how they lost their only chance of qualifying to the round of 16 to PSG, despite on two goals lead.
Literally it's not that we completely depend on the past to make out predictions, basically it's supposed to be that the past was to give us a insight of a possible outcome when certain teams meet and not an absolute to an expectation from both teams, but some persons makes it the absolute and. That could be reason for some of their losses this season. Chelsea has got a very fierce record with Manchester city of which entails mostly an epic game between the two sides , so for fixtures like this expectations usually are kept at its minimal.
I agree with @Frankolala, the past result do give an outcome of the next game and it works but not 100% perfect, since this game between Man City vs Chelsea is one of the top game this weekend in the EPL the past result can work on this but this time around things have changed and we know they played 0-2 in favor of Manchester City, in their first leg, that doesn't mean City will win again. Let's face the reality, Man City are really in one of their worse form since Pep took over and the Chelsea side are better this season and is like the both teams had a switch of form. As of last season Chelsea was suffering like City now and City was on fire, if you check what the bookies gave both teams in this encounter you'd see that City are the favorite even with their poor performance but I believe if anything concerning a win would take place Chelsea are going to be the winner, I'll be going for a 1-3 for the Blues.