What's the difference betwen Physical and Ki Defense?

What's the difference betwen Physical and Ki Defense?
And what is recommended to use?
Thanks.

Quote from luytaeyeon

Physical defense = your defense against physical damage like Crane Harmit classe skills ( ex: Crane cyclone )
Ki defense = Your defense agianst spiritualist classe and others "magic" classes ( ex: Kamehameha... )

All the skills says what are their essence....

Every Race got your "MAGIC and PHYSICAL ", Class... like saiyajin.. Spiritualist uses your basic ataque as energy, and all of their skills are based on Ki At....! On the other hand Martial Arts are physical class who can use pyshycal skills and theirs base ataque are melees.

Thanks.