chefspec: What is the difference between predicate matchers and attributes?

For example, to check that a directory has been created with the user "nobody" I can use either of two methods:

"Assert that a directory was createed with predicate matchers"

expect(chef_run).to create_directory('/tmp').with_user('nobody')

"Assert that a directory was createed with attributes"

expect(chef_run).to create_directory('/tmp').with(user: 'nobody')

There is a third method using regexs but I'm not concerned with that method.

How do I go about deciding which method to use to assert that the directory was created and is owned by the correct user?

There is no difference: https://github.com/chefspec/chefspec/blob/master/lib/chefspec/matchers/resource_matcher.rb#L32-L34

    def method_missing(m, *args, &block)
      if m.to_s =~ /^with_(.+)$/
        with($1.to_sym => args.first)
        self
      else
        super
      end
    end

The with(user: whatever) syntax is by far more common but it's up to you and your personal style.