They are very, very different things.

Zurb Email is like Bootstrap/Zurb Foundation, for email. They are common styles that apply to your HTML.

MJML, in other hand, is not HTML. You write MJML, instead of HTML, and it compiles to HTML.

It's really fantastic.

With ink (well, inky) I write an email using things like "row", "columns", "spacer" etc. and then a process converts those to HTML+CSS, then another inlines the CSS.

It doesn't seem much different to me.