Which link field accepts an URL or a Node reference? I am looking for a field just like Link, but that allows the user enter a URL or via an autocomplete a node/entity reference.

This is because this field might be an external link or an internal link in which case it should refer to the entity.

Any suggestions?
In Drupal 8 this works with normal link fields. You simply can choose if only internal, only external or both type of links can be entered in this field. You'll then get a autocomplete field that accepts either an entity title or an external link and a title.