What's the difference between the "off-contract" 4S sold in Apple Stores (https://www.macrumors.com/2011/10/15/off-contract-att-iphone-4ss-already-come-unlocked/) and the now available "Unlocked 4S" available in the online store? Is there any reason to wait and buy the official unlocked one vs. the off-contract one or are they exactly the same?