Difference between Dialog & Design dialog in CQ

Design Dialog:  It is used to globally store variables throughout  the template properties.

Dialog:  Stores all variables inside the page’s properties

One of the major benefits of using the design dialog is that if you have a hundred pages sharing the same template the variables will be shared amongst them. Also, note that you can have both design dialog and normal dialog on a page.

A dialog saves content relative to the given page.  In comparison, a 'design' dialog saves content globally (at a template level).