We just got back yesterday evening. I too was gauging the pros and cons of the I-75 route versus the I79/77 route. I got info for both routes, and ended up choosing the I-75 route for both the drive down and drive back, and am glad we took the I-75.
The advantages of the I-75 route include:
* less mountainous, and don't hit mountains until Tennesse. I79/77 more likely to have snow when you're in the mountains. We just drove back Jan 9 and 10th, and with news about bad snowstorms in the North East, was pleased to find clear roads the entire I-75 route.
* It's simple. Just get on the I-75 and go. No need to be on constant look out for the next highway - a consideration, especially in the winter months when it gets dark earlier.
* Dave Hunter's provides "escape routes", which helped me this trip. On the way down, we ran into really bad traffic south of Atlanta on Dec 26th, and having Dave Hunter's book allowed me to find a quick bypass around the slowdown).
* On speed, I am a quick driver (Orlando to Toronto took 20 hours, not including sleep time, but including all stops for lunch/dinner, washroom, rest, gas, changing drivers with DW, duty free, etc...), and found Dave's info on common radar traps and areas to be careful with speed very helpful. Aside from Dave's info, I would try to keep pace with fast "local" drivers, speed smartly (e.g. think like a radar operator -- be careful when rounding curves, coming down hills, and watching for brake lights way done the road, etc...).
This was my first "long" driving vacation with kids aged 8 and 10. Longest previous driving vacation was only from Toronto to Quebec City. With info on his board and Dave Hunter's book, the trip was very smooth. I would not hesitate to drive again.