Both of these polishes are silver holographic polishes and are Scattered holos rather than linear holos. I actually found Milani HD recently at a random CVS that I decided to stop and shop at. I was SO excited when I found it too. I already had the blue and green holo polishes from the Milani holo collection and now I have the silver, pink, and purple too! Sexagon and HD are gorgeous polishes. Sexagon is a bit more holographic than HD. HD goes on a little more sheer than Sexagon and is a bit chunkier too. Sexagon dries a lot quicker too. See if you can tell a difference.
Milani was applied to my thumb, middle, and pinky fingers. Sexagon is on my index and ring finger. Both were applied in two coats.