If you're thinking about buying an iPhone 4S, this is important: Not all iPhone 4Ses are the same. AT&T claims that their iPhone will download stuff faster than Verizon and Sprint's. The explanation is easy and logical.
The new iPhone can download at a theoretical maximum speed of 14.4Mbps, as opposed to the previous 7.2Mbps. That's because it's using HSPA+... on AT&T. (Key caveat, of course: That's a theoretical max speed, and if your AT&T service is terrible now, don't expect it to be magically a million times better and faster.)
Meanwhile, Verizon and Sprint's iPhone 4Ses still run on CDMA, which offers the same speed as the iPhone 4. If you want the download speed boost, it seems that the only option is AT&T. Additionally, Verizon and Sprint's iPhone 4S still won't do voice and data simultaneously—that's still an AT&T-only perk.