it really does depend on the project. if you are making armor, 1/4th inch is perfect, and thiner foams are good too. if you are making something ike a br, three inch foam is perfect. anything smaller than 5mm is usually for detainling rather than structure. its just preference
Yup it really is all preference and project based. I should have specified that I love 12mm for spartan builds to pass off the heavy armor look, but my next build for my wife, I'll be building wonder woman and most likely will step down to a 5mm high density give or take for a majority of that build. For an odst build, 10mm or smaller may be better as they are not meant to look as bulky as spartans.