Hi cicada!
First off: congratulations!!! Duke's program is wonderful! To answer your question: you definitely have to choose according to the faculty and department's fit to your interests. I do not think it would be a good idea to choose based on the location (and... let's face it. We are pretty much saying goodbye to our social life for a few years anyway). What I learnt from experience is that writing a thesis on a topic that does not truly interest you will make your life miserable (happened to me). If you're going to spend five years working on a dissertation, you want to enjoy it and write about something you are truly passionate about.
This being said, I completely understand how hard this decision is! Austin has a very good reputation as well and the city is, indeed, wonderful. If you could visit the departments, that would probably help.
Good luck with the decision-making process and whatever you end up deciding, the most important thing is not to regret. You got in three wonderful programs and wherever you end up going, you will love it!