If I recall correctly, once you check in, you will be able to choose a seat. Whether you can choose an aisle seat will depend on whether any of them are left, or whether you are willing to pay the fee for "premium" ones that may be available. So the moral here is if you want an aisle, check in as soon as check in opens, and hope that some aisles are still left. Otherwise, call in and pay whatever fees are required to pre-reserve. Note that if you go to a service like Expertflyer, you will be able to see what seats have already been assigned -- which may help you with your decision.