With shadow blade, you can have 4d8 + 2d6 + cha with no invocations, or 2x Cha with one. All with advantage in dim light.
With Devouring blade, you can have 2d6+cha up to three times in a turn, for a net of 6d6 + 3x cha, but only with three invocations. And for those extra two invocations you...