A login return value set by GET should have priority

(IMHO) as this enables third party extensions and site managers to develop better login integrations (they currently tend to redo custom login forms (e.g. forums, ecommerces etc. that ask a login to continue to a page) that is bad in my opinion as they should leverage on the existing login page)

Also in my notice the old $this->form->getValue('return') can be removed as never valued (i think it's old legacy inheritance, but you should check)