What about other options? 
TFDi 717
Carenado S340
CRJ
They aren't options

We try not to think in "TFDi" and "Carenado". Having nice models at hand is a part of the considerations but by far not the whole thing.
There are tons of airlines that fly everything just because a model has been released. That's fine, but it's not the way Metro operates.
Oh and by the way, the Q400 is clearly my first choice. Especially for this particular project

cu
Roman