The C series is an interesting aircraft, but i guess much will depend on what strategy Monarch have going forward, if you can fill aeroplanes and i guess monarch do then the 737 is cheaper per seat costs than the Airbus, but it would be a huge retraining effort, all the more so if they took some C series into the mix, it might work for some smaller airports but i can't see LCY being in any way relevant to Monarch and if the pilots are all on the same salary grading then loss of 54 seats on the C would need a massive saving in running and purchases costs.
It ought to be a simple Boeing -v- Airbus with the 738max & 787 -v- 320/21Neo & 330/350, i guess much will depend on delivery time tables from both manufactures, but lead times on a new order must be out to 2016/17 from both?