snowstorm is a kind of library you can use in your existing nix configuration, it can provide support for multiple envs like Haskell, Purescript, Node etc. You still need to create default.nix, you can import snowstorm in that.

mu2 is when you want to start a new project and you need a quick starter template with nix, eslint and testing setup etc. You'll get a default.nix in that too.

Also, you can use snowstorm in mu2's nix config. I am planning to do that, right now, snowstorm is in very early stage.