Sprint's model, aka the Epic 4G Touch, has that larger 4.5-inch screen, plus a slightly more rounded profile than the AT&T model. The T-Mobile version, meanwhile, is the most rounded of the three, but otherwise we were hard-pressed to find any differences between 'em, as you can see in the video below. All are powered by 1.2GHz dual-core processors, 16GB of storage, eight megapixel webcams on the back, batteries that are said to be at least 10 percent larger than the previous version, and "4G" wireless -- either in the form of HSPA+ or WiMAX.
Only Sprint has announced formal pricing and availability, September 16th for $199, while the others will be available in the always ambiguous "soon."
Update: We got curious about those battery sizes and popped the backs off the two phones that we could get our hands on. The AT&T model has the exact same battery size as the European Galaxy S II (6.11Wh), while the Sprint version is indeed about 10 percent larger (6.66Wh). We presume the T-Mob one is bigger too, but that's locked up. In a box.
Samsung Galaxy S II Epic 4G Touch for Sprint
Samsung Galaxy S II T-Mobile
Samsung Galaxy S II AT&T