Its a difficult one as there are pros and cons to both.
From a football perspective winning the champions league is more important to me.
However city winning the champions league is much more likely due to the fact it happens every year and city are genuine contenders. Most people will only see 20 world cups in their lifetime and you need a tremendous amount of luck to win it while having a good sqaud.
Winning the world cup would see the whole nation go delirious and the scenes after it would be incredible.
All in all a tough one