As promised, I did a comparison today of China Glaze IDK and Milani Hi-Res.
In the bottle I can tell immediately that the glitter and holo is going to be the difference. The base colors for both of these are just about identical.
I have 3 coats of Hi-Res on my index and ring fingers, and 3 coats of IDK on my middle finger and pinky. Here they are in the sun.
Hi-Res has holographic glitter - the individual pieces are holographic. IDK has an overall holo effect. Coverage on both was fantastic - Hi-Res covered totally in three coats, IDK only required two, but I did three for consistency. If you happen to catch a glimpse of these in the shade they are almost identical, but when the sun comes out they're definitely different.