Opposed to the International Samsung Galaxy S III which packs a quad-core Exynos SoC, the American variants — including the AT&T and T-Mobile versions we’re referring to here — pack a Qualcomm Snapdragon S4 dual-core SoC but an additional gigabyte of RAM. You could see an initial face-off between them just a couple of hours ago and, at a first glance, there is no real difference between the phones.
Samsung has made the source codes for the AT&T and T-Mobile variants public. This means that, from this point on, we can count down the days until super fast custom ROMs and improvements from the community. Search the source link for SGH-i747 (AT&T) and SGH-T999 (T-Mobile) if you want to take a peak under the hood. Development can start even before the phones are available. Thumbs up for that!