The Mirror Overfiend blade has a default damage range (stable, and the same as 90% of the weapons in AQW), and the Juggernaut version has a base damage of 0 (the most unstable damage range in the game).
My point of view is just get both if you can, that way you'll have the best of both worlds.
I bought Soul Ripper and Soul Mender; I also have Caladbolg and Mirror Caladbolg. Having the original and the mirror is twice as good as having one or the other.
(My personal opinion of course)
I feel most bad for the people who farmed the Tainted Phoenix blade; The Mirror version looks cooler ,but I think the new one has default damage range while the Jugg has a very unstable range.