I will be using the apple store app on my iphone to pre-order. During iphone 4 preordering, while the apple/att webpages were having all sorts of trouble, the apple store app cut right through that, gave me all the options I wanted, and gave me a confirmation that the phone was ordered.
For phones definately go with AT&T if you need to swap lines or do anything out of the ordinary an Apple rep can't do anything for you while AT&T might.
For instance when the 4 came out I wanted to deactivate my line add another line with the phone then switch that phone to my main line and add a feature phone to the add a line, Apple couldn't do that but AT&T did.