Both the TVs are almost equal in all aspects except price.
Xiaomi is a Chinese company; they manufacture TVs as well as mobiles and laptops. Their good refresh rate, along with MIUI optimized interface, makes it a great product. The display is better than one plus U1. However, you will get a Xiaomi TV at almost half the price of a one plus TV. The refresh rate and picture quality of one plus TVs are also very good; the smart TV interface on these phones is simple and easy to use.
OnePlus is an Indian-based brand; their latest model, one plus u1s, comes with a full HD display along with Corning Gorilla Glass 3 protection. It is an affordable product with decent features.
The advantages of OnePlus over Xiaomi are price and company support. So if you are looking for a smart TV under Rs 50000, I would suggest going with the OnePlus U1s.
Limitations of the OnePlus include poor audio quality. Xiaomi, on the other hand, has good audio quality, but if you suggest me to pick one, I will go with One Plus.