You pick out what you want when you get there.
We've done the Castle, and downtown, and we found the selection to be much better downtown. We also appreciated the fact there was more space to look around downtown. My DGD took at least 20 minutes to pick out her favorite pieces and there simply wasn't the room to do that at the castle.
Also, when we were at the castle they had limited sizes in some choices (had her dress size, but no matching shoes for her size, etc) - but DT had lots of stock in a large variety of sizes.