Yeah, except that the iPhone 4 is CDMA or UMTS/HSPA so it doesn't matter if either company has 4G/LTE coverage, the iPhone 4 won't work on it anyway.
That being said, by the time 4G/LTE is widely available, I'll be off contract, so I could go to Verizon if they have more LTE coverage. It's just that I'm satisfied for the time being with my AT&T service and coverage.
Oh, I know that they don't have an iPhone that supports 4G/LTE yet, but they will soon (another reason people would be stupid to buy one in Feb). But I'm talking in the future, all of this "speed, data/talk @ same time" stuff will be a moot point. It'll all come down to how the coverage is for all the providers in your area and where you go on a daily basis.