I haven't been in/out of Miami in ages, but I can tell you that Atlanta is pretty good and easy for long lay-overs. I've stayed at the Airport Hilton several times and it is nice. They have a shuttle which runs regularly from right outside of baggage claim. The other nice thing is if you have 18 hours and want to do some touring, Marta leaves from the airport and takes you right downtown to some decent restaurants as well as places like World of Coke, CNN Center, Olympic Park etc.