For most instances, Shadow Image > Dark Shades.
It's hard to beat an ability like Shadow Image that 1) maims a target from range, and 2) allows teleportation back to said target in an instant - which is why Dark Shades is lackluster in comparison (it just summons two shades that both debuff the same target, and have to do so at melee range while providing very little damage themselves).
I have been thinking about ways to make Dark Shades a more desirable morph, and wonder if it would benefit from being a "cast and forget" toggle like Leeching Strikes. In theory, if implemented, the Dark Shades morph would be up at all times as long as it is toggled on and slotted on your bar. Tanks would benefit from not having to recast Dark Shades over and over (most use Refreshing Path to activate Shadow Barrier, anyway) and it becomes a little bit more desirable of a skill to at least consider, instead of Shadow Image.