Now that the iPhone has made its way to three of the four major carriers, the game to differentiate is officially on. While customers of AT&T, Sprint and Verizon who choose to buy an iPhone 4S will all be getting identical devices, the experience they have will differ due to the nature of the individual network.
For starters, the new iPhone was designed to take advantage of the not-quite-4G HSPA+ network for faster downloads. But of the three carriers, only AT&T actually offers HSPA+. And since the 4S can't run on Verizon's LTE network or Sprint's WiMAX, customers of those carriers will be stuck with plain old 3G, just as with the iPhone 4. AT&T's network is also the only one that will allow you to talk and use data at the same time.
And then there's the matter of data usage. The iPhone is a well-known data hog, and partly due to its ascent in recent years, both AT&T and Verizon have stopped offering unlimited data plans. At AT&T, your iPhone 4S will be capped at 4GB per month with the top tier plan, and at Verizon the ceiling is 10GB. Sprint however, has said it will offer a truly unlimited data plan for its iPhone customers, though there's no word yet on price.
Verizon customers won't get fast or unlimited data like those at AT&T and Sprint, but they can always console themselves with their number one-rated service coverage.