IIRC, you can use either. Whatever quote you used to start the string is the one that will end the string.
var foo:String = "foo"; // foo
var bar:String = 'bar'; // bar
var fooquotebar:String = "foo'bar"; // foo'bar
var foo:String = "foo"; // foo
var bar:String = 'bar'; // bar
var fooquotebar:String = "foo'bar"; // foo'bar
var z = "This is just a literal value";
var q = 'This automatically runs Std.format, letting me include "$z"';