AT&T or Verizon?
Both the GSM and CDMA iPhone 4 offer almost identical specifications. Both have iOS 4.x, 5 MP cameras, 1 GHz processors and 512 MB RAM. Both are priced at $199 for the 16 GB version and $299 for the 32 GB version.
The AT&T iPhone 4 has no unlimited data plans though, while the Verizon iPhone 4 does. While AT&T offers 2 GB data for $25 with $20 extra for tethering support, Verizon has an unlimited data plan for $29.99 with $20 extra for tethering.
The AT&T iPhone will have global roaming support, but the Verizon iPhone 4 won't. Both offer 3G connectivity, but the Verizon speed advantage won't work there. Neither phone supports 4G connectivity, and AT&T's 3G is faster than Verizon's.
AT&T iPhone users will also get upgrades earlier than the Verizon users and they can also use simultaneous voice and data, while Verizon iPhones don't support data and voice simultaneously.
But Verizon users will have one advantage. They will have the Wi-Fi Mobile hotspot option while AT&T users won't. The battery life is likely to be the same on both iPhones.
Also, if you enter a Verizon contract now, by buying the Verizon iPhone 4, you may not be able to upgrade to the iPhone 5 when it launches.