There is no difference really. The Rails doc says this about head:
The head method can be used to send responses with only headers to the
browser. The head method accepts a number or symbol (see reference
table) representing an HTTP status code
head :ok sets render to return just the header with status 200.
It’s merely a shorthand for render nothing: true, status: :ok.
Rails 5 will also do head :no_content by default when you don’t have a template defined for an action