Go to an AT&T store.
You can buy it ahead of time, then pick up YOUR phone on the 19th. (Not, first come first serve, but an actual reserved phone.) And since you've paid ahead of time, I imagine it will go faster than at the Apple store, where you haven't paid yet.
Now, it's possible AT&T won't get all their shipments on the 19th, but since there's a specific phone in your name, you can call the AT&T store and ask if it's there or not. They'll say 'yes' or 'no.' The Apple store can't answer that question ahead of time.
So neither way is 100% sure, but with the Apple store you have to stand in line without knowing what's going to happen. With the AT&T store you can at least check if your reserved phone is there or not before you go.