I've checked the whole site twice, and the best I can tell is that the only difference for the Dungeon Master edition is that it has different box cover art. That doesn't seem worth the upgrade to me.
Yeah, that seems to be the only difference BUT that's all they promised on the pledge page for the Dungeon Master Edition.
"A Kickstarter exclusive box cover featuring the Dungeon Master, plus all the unlocked stretch goals."
It was $5 Canadian more and I'm happy with the different box.