Tibet is a plateau region with relatively high altitude and thin air. Most people will have altitude sickness when they go to the Qinghai-Tibet Plateau, and even their lives will be in danger. Therefore, students with poor physical conditions are not recommended to go to Tibet.
There will certainly be no high reaction in Xinjiang, but the temperature difference in Xinjiang is relatively large in the morning and evening, and it is particularly hot in summer. Xinjiang's melons and fruits are famous all over the country. If you go there to teach, you should have a chance to taste the fruits and vegetables in Xinjiang. I personally suggest teaching in Xinjiang.
No matter where you go, the conditions must be hard. You must be mentally prepared and pay attention to your words and deeds. Take children seriously, they need you!