Another thing to consider is this ... how far north are you thinking of going? While Florida might have beautiful weather in December, as you travel north, your likelihood of encountering snow increases. Just something to keep in mind.
The seat option, rather than the sleeper option with the train wasn't too bad when I rode the train (either that or I was just so happy to finally board the train!!), but the biggest difficulty was the delay. If you've only got a short vacation, you could spend one entire day just sitting in the station.
Yes, European trains are MUCH more efficient and MUCH less expensive!! Even British trains, which the British complain about all the time, are more efficient and less expensive.