To be completely honest with you, if I was completely new to the C-Drama realm, I wouldn't even start with any of those 2 yet.
I would start with the modern and easier to follow ones. Maybe "Old Boy" or "Ode to Joy" for lighthearted slice of life, or "Love 020" if you like cute romance, or "Love me if you Dare" if you would like a more thrilling one. Once you've eased into it, then start picking up the fantasy and historical ones.
But it's all your choice. Should you still wanna start with one of the 2 you mentioned, then definitely start with TMOPB. Nirvana in Fire is amazing, but it's much much heavier :)