Consider what the dangers are.
Both contain blackpowder. Both are suseptable to ignition. Steel protects from both sparks and humidity.
Any container, when the contents ignite becomes the material that flys through the air. That's a risk. Balance the two risks - make a choice.
At the arsenal (Radford Army Ammunition Plant - 20 miles down the road) BUILDINGS are made with thick walls and easily blown-off roofs. That is to protect people. Ammunition storage is in widely separate bunkers, kept cool and dry.
There are specific guidelines for storage of blackpowder, smokeless, fuse, primers et al. published. I don't remember the location, but when someone surfaces with it, I'll put it in the REFERENCES sticky.