Well, the master of commerce is just a intro commerce degree for people who wants to change specialization. I think its just subjects taught at a slightly faster pace compared with the bachelor version. So it is just a mix of masters/bachelor, since it is a degree for say a person who done arts but now wants to major in finance.
If you are a finance major, you wouldn't do master of commerce and then major in finance again since it's practically the same thing as you did before. So you would pick the master of finance, which is designed for finance graduates who wants to learn more.