Well we've done all three. Flying is quick, and with sw the price is great even with a family of five.
I always have enjoyed a good road trip, so i like driving too, enjoy stoping in DC to visit a museum or two and the monuments. Have also stopped in Charleston and Hilton Head, Have driven 95, 301 & 17. didn't mind any of them, well okay i'm not crazy about the turnpike in New York/New Jersey. If you leave at the right time, you can avoid almost all prime time traffic. Take your time, take alot of short rests, and stop when your tired.
The Train, DS#1 and I took the Auto-train from Sanford, Fl to DC last Dec, i really enjoyed that experience. Found it very relaxing, we had a sleepette (the smallest sleepers) Left the DC train station around 9am and were home in Western Mass before 6pm. I would do this again anyday. My 19 year old son is still whining about this- he hated it. If i get the opportunately to do this again, i would try coach to save a little money.
Back in 89 i took amtrak from Sprinfield, Ma to Ocala, Fl (actually only made it as far as DC due to a derailment & missing connection). That i did not enjoy, way too long of a trip to be sitting in a coach train, the staff were soo rude, and the most unhelpful people i have every come across, and you got to the what i love to call "the arm pit" of the east, the train does not travel past the most beautiful parts of the east coast. The Auto-train is a much nicer train.
Let me know if you have any questions
deb