Did this the other day:
I don't usually like to use words like "better" or bluntly state that one amp is better than another... I think everything has it's place, y'know? But in this instance, I think the Micro Dark is simply a superior, refined version of the Micro Terror. More usable sounds. The Shape knob has no bad settings, where the Tone knob can go too flubby or too shrill. Every setting on the Micro Dark is useful whereas the Micro Terror has a "sweet spot" you don't really want to stray from.
The Terror is like an old school 70's Marshall or similar beast in that respect... It does that midrangey distortion tone all day without slouching. The Dark is more like an Ampeg V4 or my Orange Overdrive where the amp has a ton of different usable sounds and reacts the way you'd expect it to as you tweak the knobs. I would recommend both but I prefer the Dark and I'll pick one up soon as a back-up for some gigs I have coming up.