At my feed stores mash is well mash just looks like ground up food and they maker layer in a pellet no layer crumble....my spoiled chickens do not even like little tiny pellets only like crumbles or the mash but they love any kind of seeds or grain except whole corn and any kind of scraps thrown out my spoiled weird chickens....
You can get Layer in -
Mash, Crumble and Pellet. I believe that most people like to use a Layer around 18% protein (some use a little more protein)
I myself don't feed a layer I feed a Game Bird Feed and offer oyster shells.
The ingredients for a feed ration are mixed, ground, mixed with water and extruded into pellets. The pellets are baked at a high degree to eliminate moisture and kill pathogens that naturally occur in grains (usually various forms of mycotoxins). For crumbles, the pellets are coarsely ground; for mash they are finely ground. For pellets, well, the pellets are not re-ground.
I've always preferred pellets for the simple reason they make less of a mess--mash on the floor gets mixed with poop and makes a crusty layer. Don't know if my chickens have a preference since I've never given them a choice.
My birds seem to prefer the pellets, and I do too, because I like the low dust content. But if I had broodies and chicks, I'd definately use a mash. Chicks can't break apart the pellets.
One other consideration, at least in my area, is price. Locally, crumbles and pellets come from Purina or other such manufacturer. The mash, however, is produced locally and therefore much less $$. A 100# bag of mash is ~$18.00. A 50# of crumbles is ~$14.00.
Yes, I am a cheapskate. But, I mostly chose the mash b/c I prefer to spend my money locally when possible.
It is definitely dustier as some have mentioned. Honestly, they're chickens. They poop on everything. I don't think the dust is a big problem.