We chose Miami because it is on the East Coast putting it in the middle of travel for West Coast US, East Coast US, and European travelers. Miami was chosen specifically because it doesn't have the weather issues that arise in the back portion of the year that are part and parcel of living on the East Coast. It could be 100 deg or 3 ft of snow, it is anyone's guess for most cities capable of hosting a conference in fall. Miami was also chosen because it is not the "goto" place for conferences so it will be an experience, which is what we are working at setting up. Also the fact that it is not just a commuter event means that those attending will be there for all of the social events without the strong urge to run home for the evening.
As for differentiation, we love what the PSF is doing with PyCon. Our target is a smaller, tighter focused, more intimate experience. From top to bottom the event is focused on showcasing where things are going within Python, so talks will be far more "risky" and "futuristic" than the average PyCon talk. To clarify, this is not a dig on PyCon, just a differentiation point - both formats are needed. The conference itself will have a more social feel about it than other conferences with fully planned out schedule including evening social events (see CodeConf or JSConf for models). Also with PyCon 2012 moving west, PyCodeConf is a great way to get your Python fix if you are on the East Coast.
Hopefully this helps.