My last 3 trips, I had done connections in Atlanta airport, and it is a wonder. It is very easy to switch from gate to gate or terminal to terminal... Normally, my flight arrive from NJ on Terminal A or B, then I just go down, take the train to Terminal E (where International Departing usually is) and done. It will probaly takes no more than 15 minutes.
On the way back, when you arribve (from International Destinations), you have to do Customs, but it has not taken me more than 45 minutes to do Customs, pick up baggages, then drop them back, and arrive to the next gate.
My last experience was on April 30th, coming from Cancun, Mexico to Newark. We arrived at 4:10pm and by 5:10pm we were at the next gate (we took longer because our Gate was not ready - another plane was late and was at our gate - so we sit on the plane for 25 minutes, then everything was quick, oh before I forget, also at immigration we have like 3 other planes arriving at the same time)..
Final Opinion: ATLANTA 100% over Miami..