Isn't one of them shimmery and the other has a matte finish? If that is true, which one is which? My instinct would be to say the luster is the shiny one, but I figured I should double check first

You are correct. Luster dust has a shimmer to it, and petal dust is flat color.

Thanks for the response! I want to order some colors and just wanted to make sure I don't end up with the wrong stuff