Napa leather is a leather, typically dyed, made from kid-, cow-, calf-, lamb- or other skin by tanning. It is noted for its soft temper. It is a generic term in the leather field and has no distinct test for characterization. Because of this ambiguity, the term is often used in advertising to imply that a leather has a soft hand. Among other uses, Napa leather is often used in leather products such as furniture, clothing, handbags, and shoes. As the term is used today, Napa Leather may be either natural grain, or more likely, corrected grain.
Wikipedia

Mulan.....can't find anything definitive.

 

Go see some cars of both. I bought 2015 NAPA. From what I could tell the seats are the same and the difference was the doors and dash. Noticeably softer in those areas. Also when I ordered it a year ago the packages were different so pay attention to what pieces are what color. But then you said probably black. Basically I don't think it was that big of a difference.

 

This has been discussed at considerable length here. Do a search and you will have your answers.

 

Much has already been answered above in previous posts (thank you). Addressing only the issue of durability, GM did the same and extensive testing of both leathers, and determined that both the C7 Napa and Mulan meet the same durability standards.