Well, if it were me, I'd go I-75, straight through Atlanta, (too many trucks on I-285). Stay in the dead middle of the freeway in Atlanta to avoid disappearing lanes. Take I-75 all the way to the Florida Turnpike at Wildwood, take the Turnpike to Fl St Rd 429 (toll) south to I-4 west, I-4 West to US 27, US 27 south until you hit I-75 again, west of Ft Lauderdale. I-75 south to the Fla Turnpike extension. South on the Turnpike to US 1, and US 1 to Key West.
From my experience, there is usually less traffic going straight through Atlanta than dealing with the trucks on the I-285 perimeter. I take the Florida Turnpike to FL 429 to I-4 to US 27 to avoid the traffic, traffic lights and congestion around Clermont FL on US 27. For a few bucks in tolls you save about 1/2 hour. I'd go US 27 instead of the Turnpike all the way because the traffic on the Turnpike can get really crazy from Palm Beach through Miami. On US 27, you hit a few traffic lights but it is good 4 lane divided non toll road all the way to I-75. You could stay on US 27 to the turnpike in Miami, but, I'm not sure about the traffic closer to Miami, and I prefer I-75 to the Turnpike Extension. From Ft Lauderdale through Miami, expect lots of traffic and erratic drivers. JMHO
Well, that's how I would go, others' opinions may vary.