Willy, you might owe exDubai an apology.
CityJet did choose the Sukhoi SSJ100 over the CS100. CityJet supposed to be a "shoo-in" for the CSeries because they were going to buy CRJ900s for SAS and Bombardier could put together a "package deal".
In fact last year there was open speculation that CityJet might fly an all-Bombardier fleet:
Bloomberg: Bombardier CSeries Order Stoked by London City Specialist
This was a "head-to-head" competition between the CSeries and the SSJ, and even though the CSeries was favored, the Sukhoi won. CityJet ordered 15 SSJs, with an option for 10 more. Then just this week they converted those 10 options into firm orders, and added another 6 options.
The CS100 can take on more passengers and fly further, but the SSJ100 was likely offered at near half the price. The CSeries simply could not compete with that kind of pricing, with little chance of making up the difference from operational savings due to falling oil prices.
Even with the weak Canadian dollar, the CSeries is still not competitive. That's why yesterday Bombardier publicly signaled that they are now willing to offer the CSeries at heavy discount:
FP: Bombardier sees ‘aggressive’ CSeries pricing for new customers amid 15-month order drought
Rumors say this time they are having a tough time competing against the Embraer E2 for a possible United order. I mentioned on the other thread months ago that Bombardier should consider selling even at an initial loss, and if you read Fred Comer's comments in the above article, it appears they are now prepared to do so.