No airline that I have ever heard of will give you a choice of aircraft. Your entire class may be assigned one aircraft, or they may split the class between aircraft. In the later case, they will usually ask you for your preference, but will satisfy their own needs first, then go by seniority. For example, high-time folks will be assigned to aircraft which are expected to have rapid upgrade, to ensure that they have enough captains available down the road. Also, I know SKW has a tendancy to hire CRJ drivers from other airlines, and obviously assign them to the CRJ. Other new-hires tend to go to the EMB.