If you are looking for the train "experience", then you will definitely have it in the head-seat class. If you are not in it for the experience, then take other posters' recommendations and fly. Save lots of time to fly (but pay more though).

The station in Xian is more of a mad house (MHO) than that of Shanghai Station. Finding the end of line for the taxi is overwhelming (even for repeat trips). I wouldn't want to take the bus there. My experience was based on non-holiday period travel.