Got a little of everything to be honest. Never felt the need to be that organized with loaded ammo seeing as it is supposed to go right into a firearm and downrange!
I use the plastic ones when I have empty ones. Hate Styrofoam. Like fingernails on a chalkboard.
For short runs though, I use ziplock baggies. Sandwich ones mostly. I write on them with a Sharpie to tell me what the load is. 30 rounds or less.
I also use empty powder containers, best use of recycling that I have found, and empty coffee cans (plastic).
I haven't gotten to the 5 gallon bucket yet but some of my competition rounds might fit in them better!