While still a long while a way and a lot can change, what factors influence choose equipment if a base you desire has multiple. For instance, if one base (not sure if this exists) has all 3 birds and you have no personal preference over the machine, what else do you consider? ORD has both CRJ and ERJ and if given the choice, I don't know what would make me choose one over the other.
Go with the EMJ if you want more flexibility + reserve time seems to be less than the CRJ.
Go to the CRJ if you want a bigger airplane.
Go with the ATR if you want QOL and will get for sure a line faster than the EMJ and CRJ.