Which is more preferred professionally between a resume and cover letter?
Cover letter is a letter than you address to recruiter which contain a brief detail of your professional summary whereas resume contain detail of your professional career. Both and important but cover letter and resume are not substitute of each other. Both have different purpose
Most companies do not need a cover letter, so resume is more important in these cases.
However, in cases where a CL is required, it has a lot of importance since it is an opportunity to talk about your motivation for the role.