Here's what I'm starting to observe...

I used the liquid dye and ended up having a hard time getting it to go black.

People using the powder end up with black no matter what.

So... let's see:

Powdered dye:
+Shorter dye time
+Great for dark colors
-Requires buffing

Liquid dye:
+Light colors are easier to hit
+No buffing required
-Long soak time with extra heat
-Hard to get pure black