For me, the feeling of these dramas was the same. Nirvana does not have romance, but love is used, sometimes, in Empress Ki as mean to a purpose.
You cannot get bored at any if these two dramas and if you like strategy and court intrigues dramas, make sure you watch both. Great leads and supporting actors/actresses in both, stunning visuals, suspense.
I do believe Nirvana was better acted and characters were more complex, but if you liked one of these 2, don't miss the other one.