Hi everyone, I'm trying to decide between 3 different types of AL powder for a flash composition. Their may or may not be a real big difference between the three but I'm curious to know what everyone thinks the best kind would be to get..

ECKHART 5413-H GERMAN BLACKHEAD 3 micron
ECKHART 10890 GERMAN DARK 6 micron
INDIAN BLACKHEAD 4-5 micron

All three will do just fine

the 3 micron blackhead will burn the fastest ... technically.
IMHO, you'd have to mix it very intimately to notice the difference between 3 and 6 micron, I'd just get the cheapest.

yeah, I think by that point the limiting factor is going to be the size of your oxidizer particles, not the fuel. For future reference, anything labeled "flake" will burn faster than an equivalently-sized powder labeled "spherical." Not that faster is always better.