Both are based on the China dynasty period.
The difference is that, TSOMD is mainly a detective show where they solve case by case but eventually saving the emperor from rebellion. It focuses more on the thing that happen around the royal Palace.
As for nirvana in fire, they focus more on the things that happen in the royal Palace, and the main character is trying to help the Prince becoming the next emperor. You'll see the details about how the emperor works, their concubines and the way the Princes are trying to get their hand on the throne.
Both are equally nice, tho the goal is different.