We were told that cheap flights in China are the way to travel when you are in China the thing to remember is that China is huge and the times for the trains are measured in days. There are several regional airlines in China that operate flights, the national carrier Air China & China Southern operate internationally as well and would probably be the 2 biggest, and then you have Sichuan (SIC) China Eastern, Xiamen Air & Hainan Air. For the best individual hubs you will find generally the cheaper airfares for instance the cheapest airfare from Xiamen to Xian was on Xiamen Air and the best price for a flight Lhasa to Beijing was on Air China .
For trains, the only time that I would recommend them would be the train to Lhasa and then I would recommend that you only go from Xian, and not from Beijing, which takes 12 hours off the trip I would also suggest that you only plan to go in one way as the trip is long and the view going in the other direction is pretty much the same also if you are suffering from altitude sickness you will get back to sea level sooner. The flight that we got to Beijing stopped in Cheng du which was nice to see from the air and the airport was very nice as well.