I have often used the train during the day in other parts of China, but I wouldn't advise the overnight train. The first hard part is negotiating the station, where touts and pickpockets are common, then securing your luggage onto the train. This is not too bad if you have Chinese speaking friends who can help, or a busboy from the hotel. Friends of mine were robbed last Friday night on an overnighter to Beijing. They lost their passports, credit cards and cash, and the scary part is that not one of them noticed until morning.
Flights are cheap and there are plenty of them.