This weekend I finally decided to swatch them to see how different and/ or alike they were. I did two coats of each nail polish and I only took sun pictures because the lighting in the shade didn't show any of the glitter.
This is Sinful Colors Secret Admirer. If you're looking for a black polish with noticeable silver glitter, then this is the polish for you. This was the only one that you could really tell there was glitter in there and it almost gives it a duochrome look.
This is Little Black Dress. The L.A. Colors one and this one were the only two that came close to what I envisioned black with microglitter polish to be. They are both black bases and both have microglitter, but it's less pronounced and gives it a more subtle look than Secret Admirer.
Here is L.A. Colors. Unfortunately, it didn't have a name. I bought this one in a set of six other polishes. The packaging said it was the Shimmer Brights Collection, yet I couldn't find any names for them online. This one and Little Black Dress are pretty close dupes.
The last one is Essence Fabuless. This polish is definitely way more subdued than all of them. It's basically a black creme with a sparse sprinkling of microglitter
I'm sorry that I didn't provide full swatches of these, but my original plan was to take a closeup of them all together, so that you could distinguish the difference better. However, that was a total fail. When I took pictures like that, you could barely even tell them apart, except for Secret Admirer.