An engineer at work told me that because "+" uses two channels to get its extra speed, that 80% of towers have the feature disabled, to avoid exhausting their capacity.
Most people think that AT&T has faster speeds; and I assume if you're right next to a tower, it's true. But take a step left or right and you find yourself in a gap, then all the data alphabet soup is meaningless.
I can only say this about my particular area. But the fact that consumer research has found that after evaluating 23 cities, AT&T comes out worst by far is no surprise at all.