City is a better team, I have no doubts. The best in the world, only Bayern could rival them. But Chelsea has the potential to be an immensely tough nut to crack against any top team. Not only their defense and collective positioning off-the-ball are amazing, but they are also very physical, can be dirty, disrupting the opponents's rhythm with countless harsh fouls, and have amazing counter-attacks. The perfect set-up against any proactive team like City, no matter how good.
And Guardiola is no exception to struggling against such teams. 2009 is a good template. That Chelsea's playstyle also had those characteristics I described. Barcelona drew 0-0 in the first leg, the only game in that season in which Barcelona failed to score at the Camp Nou. And the second leg was a 1-1 draw. Even if we don't take in consideration the penalties not given to Chelsea, the fact is that Barcelona had over 70% of possession, but couldn't do anything with it and were completely limited to shots of faith from outside the box.
Chelsea, meanwhile, had at least four amazing chances to make the game 2-0 and kill the tie, but wasted them all. All in fast counter-attacks. Iniesta punished Chelsea with a goal in minute 90 of the game, which was the first and only shot on target by Barcelona in the entire match, they previously hadn't hit even the post or the bar, had no real goal chances. Guus Hiddink really deserves a lot more credit.
Barcelona's tough games against Chelsea also led lots of english football fans and press to claim that Barcelona and Messi were tremendously overrated and overhyped. They said that United would easily win the final.
I can see the current Chelsea in the right day pulling off against City something like what Chelsea did against Barcelona back in 2009.
Guardiola's history against Chelsea in the UCL:
Barcelona 0-0 Chelsea, 2009 UCL semifinals.
Chelsea 1-1 Barcelona, 2009 UCL semifinals.
Chelsea 1-0 Barcelona, 2012 UCL semifinals.
Barcelona 2-2 Chelsea, 2012 UCL semifinals.
Guardiola in the post-match interview after the second leg against PSG:
"People think you have to be in the final every year. It's unfair, we won the first game because we shot between the hips of the opponent, Man Utd won one because John Terry slipped, Madrid won against Atleti in the 93rd minute, it's the little details."
I don't think that him using United, Chelsea and Real Madrid as examples was a mere coincidence...