Asking hiring managers if they think the resume or cover letter is most important won't likely produce a substantial qualifying answer, much like if you polled 100 random men whether they preferred big boobs over nice legs/butt. It is just a matter of preference.
*Although the correct answer would be legs/butt > big boobs, and always complement your resume with a solid cover letter.
:v_SPIN:
In the words of Jerry Seinfeld, I have legs.