Beijing - Xi'an Train vs. Plane

The land distance from Beijing to Xi'an is about 1,200 km (746 mi). The door-to-door travel time takes totally 6.5-8.5 hours by high speed train and 6-6.5 hours by flight. If you want a faster transfer, choose the flight; if you want to enjoy the land scenery and high speed, then choose the high speed trains.
 

High Speed Trains	Plane
Journey time is 4.5 - 6.5 hours.	About 2 hours onboard.
CNY 824.5 for first class seat; CNY 515.5 for second class seat	CNY 1,850 for full fare economy class; but usually 20%-60% discount are available
Punctual	Possible delays
Mobile devices are allowed to use	Mobile devices switched to flight mode
Beautiful land scenery	Blue sky and bird's eye view of two cities
About 17 departures per day.	Over 40 departures per day

 It is true that a flight is shorter than rail travel, but you need to arrive at the airport 2 hours in advance to board the flight; also, airports in both cities are a little bit far away from the downtown area. If travel to the airport and boarding time are added, rail travel time is not really longer than flight travel time.
 See detailed Beijing - Xi'an High Speed Train, Beijing - Xi'an Flights

 Note: Time consumed by traffic jam and flight delay excluded.
 Aside from high speed trains, some normal trains traveling overnight are also good choices. Traveling overnight saves day time and one-night accommodation fee. The sleepers onboard can ensure you a good rest.
 See detailed Beijing - Xi'an Overnight Sleeper Train
 A second class seat on Xi'an to Beijing high speed train costs CNY515.5, while the full price of an economy class seat on flight costs CNY1,850, which is even more than a high speed rail trip in business class. Of course, you may get good discounts when buying air tickets in low seasons; in such a case, an economy class flight ticket may cost CNY300 or so and therefore will be more competitive.
 This is owing to air traffic control backup, smog and weather changes. In 2016, the delay rate at Capital Airport was 39.3% and 17.7 % for Xianyang Airport. In contrast, trains are well scheduled, and usually adhere to their timetable.
 You can enjoy the window view and more legroom when making a rail trip and move around freely; and you can also surf online with your mobile devices. On the other hand, mobile phones need to be switched to the flight mode on airplanes and cellular data is forbidden. Bigger PEDs like Kindle and iPad are not allowed to use during takeoff and landing.

Mobile devices can be used onboard.

Mobile devices need to be set to flight mode.

 About 40 pairs of flights are in service per day; but only 23 pairs of trains are available for your convenience each day, among them, 17 pairs are high speed trains and the other 6 pairs are overnight sleepers ones.
 Traveling along Beijing to Xi'an high speed railway, passengers will be able to appreciate the scenery between the North China Plain and the Central Shaanxi Plain; the famous capital city with traditional and modern elements, the agricultural areas, the undulating terrain, the rivers, and the vast plains will all make your trip more attractive and wonderful. In contrast, the flight trip will only award you a bird’s eye view of the two cities and the blue sky, white cloud and bright sun.

Further Reading: How to Travel from Beijing to Xi’an