It would have been difficult to predict if both teams are on their peak form but just like you said, they are both playing averagely now and thereby making it easy to predict Manchester City will win. No doubt, Manchester City have faced some serious set back and declining performance in various previous games but they seem to have recover or recovering having a convincing win with high score line against Ipswich Town during the weekend. PSG is playing in a less tensed league and still struggling in the midst of teams that will have no place to play in Premier league. Despite City bad form, they'll probably be champions suppose they are playing ligue 1. There's no certain prediction in football until the final whistle but city have high chance of winning here.
In some games City are already able to score as before, despite the fact that Ipswich can be considered a weak team, but they rarely concede so much, and City increasingly manages to score a lot of goals in recent matches. PSG play more modestly, but their affairs in Ligue 1 are much better, and it is unlikely that anything will prevent them from becoming champions. In the Champions League, they are not all right, I think that City will play more carefully because this is an away game, and for PSG in this match only a victory is needed, this is what they will try to do.