It depends, really: if you are working on data that might be an array in one case, and an object the next, it would probably be best to use the json_decode
trick, simply because unlike a cast, its result is "recursive". There is one very important thing to keep in mind here, though: numeric indexes can, and probably will cause problems for you at some point in time. Take a look at this bug report
This is documented here, but not in a way that really stands out:
If an object is converted to an array, the result is an array whose elements are the object's properties. The keys are the member variable names, with a few notable exceptions: integer properties are unaccessible;
Exampe of the problem:
$data = [
'foo' => 'bar',
123 => 'all is well',
];
$obj = json_decode(json_encode($data));
var_dump($obj->foo);//bar
var_dump($obj->{123});//all is well
$cast = (array) $obj;
var_dump($cast);//shows both keys
var_dump(isset($cast[123]));//FALSE!!!
var_dump(isset($cast['123']));//FALSE
Basically: If you start converting arrays to objects and back again, numeric keys are not reliable anymore. If I were you, I'd simply change the code that is passing the data where possible, or I'd create a value object that can be set using an array or an object, and normalize the data that way.