I suspect it won't make much differeance in the end as both stores _should_ be properly equipped, trained, and setup to handle any customer. If you think your situation is unusal in some way it won't hurt to stop in to a store the day or two before hand and talk wit the sales people and then decide where to purchase based on their answers.
What you need to ask yourself is if you an existing AT&T customer? Are you a new customer? Porting an old number?
The answers to those questions whould tell you whether to go to the Apple store and an AT&T store.
Personally for myself I will be porting an old number which I suspect might be a little unusual for the Apple store folks to handle so I will be going to the an AT&T store. I have three stores already identified that are somewhat near me. On Saturday the 23rd I will be visiting all three stores, talking to the sales reps. I will be looking for a nice friendly rep who seems to know that they are doing. I will get copies of all the paperwork to take home and fill out ahead of time. I will make a big enough impressions that the guy remembers me the following week. Thats my plan anyway.
- SR