Perception: Verizon is superior to AT&T, so their iPhone is the better buy.
Reality: Verizon voice calls are better, but their data network is inferior.
This according to the WSJ’s Walt Mossberg:
What about the trade-offs? Chief among them is data speed. I performed scores of speed tests on the two phones, which I used primarily in Washington, and its Maryland and Virginia suburbs, and for part of one day at Chicago’s O’Hare Airport. In these many tests, despite a few Verizon victories here and there, AT&T’s network averaged 46% faster at download speeds and 24% faster at upload speeds. This speed difference was noticeable while doing tasks like downloading large numbers of emails, or waiting for complicated Web pages to load. AT&T’s speeds varied more while Verizon’s were more consistent, but overall, AT&T was more satisfying at cellular data.
Also, because Verizon’s iPhone—like most other Verizon phones—doesn’t work on the world-wide GSM mobile-phone standard, you can’t use it in most countries outside the U.S. AT&T’s iPhone does work on this standard, and can be used widely abroad, albeit at very high roaming rates. In the midst of my testing, I had to travel to Hong Kong, one of the few countries where the Verizon iPhone functions. But even there, it only worked for voice, not data, at least in the areas where I was working. The AT&T model handled both voice and data everywhere I tried it there.