What is the difference between duo and and mirror

What is the difference between duo and mirror other than a couple of hundred bucks?

How about between duo and EX2

Seems like they are all 2 bay nas units.

Hello, you can check the information page for both of them to have a quick look at the major differences. 

My Cloud EX2

http://www.wdc.com/en/products/products.aspx?id=1180

My Book Live Duo

http://www.wdc.com/en/products/products.aspx?id=620