I prefer the "canon" version, because it's thematically linked to Mirror Mirror and Spock's actions. I hated in Dark Mirror that they basically swept him under the rug. The point of the episode was that a man can change the present... only well, apparently, he can't. Alas.
In the canon version, on the other hand, Spock's actions did change the future... for the worse, in true Mirror Universe's style. So, very apt. Also, the fact that Terrans got to taste their own medicine was the only way to eventually redeem them, which makes for an interesting narrative arc.
Dark Mirror was fun to read, but it didn't have any resonance beyond the gimmick.
Dark Mirror didn't sweep him under the rug, though. They had him continue and profess reforms only to be killed after being framed by Kirk.
My problem with the Terran collapse version of things is that it basically said the Empire collapsed because of democracy. That that very thing which Kirk was espousing as something to better this universe, and save their nation, actually lead it to total collapse. That, and the Terran Empire is fun. Who wants to see it go, and get replaced with a who gives a fudge menagerie of people. I don't much care about civilian Sisko, or whatever he was. I care about Captain Sisko, commander of the captured station Deep Space 9, with a sash around his waist and some battle scars from the conquest of Cardassia Prime. That's what I think is awesome.
But they didn't do the Evil Universe thing much. DS9 never had any version where the Terran Empire was still around; there's was the version where it collapsed, and the Terrans were now rebels trying to kick the Cardi-Klingon alliance out. We only had two episodes that involved the Terran Empire, and they were amazingly fun.Eh. I mean, you can only do "Evil Universe!" stuff for so long; afterwards, it just becomes boring, as DS9 proved.