Taking a train to Tibet is the best choice for travelers who have never been to a plateau because it's prone to get altitude sickness if flying directly to Lhasa. If time is tight, you can take a plane on the return trip. In addition, you can enjoy much more plateau scenery by train than by plane. There are snow-capped mountains all the way, green grasslands, and wild animals. From time to time they encounter Tibetan antelope, yak, black-necked cranes, vulture, musk deer, and marmot, etc., which you can't experience by flying.
PS. The scenery from Golmud to Lhasa is the most beautiful, and it would be better to arrange your train travel during the daytime for this section. Tibet trains depart from popular tourist cities like Beijing, Shanghai, Guangzhou, Chongqing, and Chengdu, where you can also stay for a few days for sightseeing.