Pass object or id to a Model class?

Let say you have a model class Blog, and has a method getPostsByAuthor

function getPostsByAuthor($user) {
    ..
}

$user = User::getById($user_id);
$posts = Blog::getPostsByAuthor($user);

Assume $user is another Model class called User, there is another way to write this method, e.g.

function getPostsByAuthor($user_id) {
     $user = User::getById($user_id);
     ...
}

$posts = Blog::getPostsByAuthor($user_id);
// or if $user already exists
$posts = Blog::getPostsByAuthor($user->getId());

It seems to me the 2nd way of writing lead to a more re-usable code and it preferred? Right?