We have flown many times through both. All in all, I really cannot pick one over the other. They are both large terminals.
Yes - Miami is larger. The walk from low-numbered American terminals to high-numbered is long, but they have a Sky Train that makes that much shorter.
Charlotte has no Sky Train, and your flights may be from different concourses, so the walk can be longer than Miami.
The Miami terminal has much better food selections.
Honestly, it is not worth an additional two hours to go through Charlotte.
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