Once you share how you'll be pursuing your next exciting career role, I'll be able to advise you on the best resume supplement. For example, if you're sending your career docs mainly to recruiters, I may suggest a savvy e-note, but if you have a little bit of a story to tell or work in a more traditional industry, then it'll be cover letter all the way.
Now, as I'm sure you have, I've heard that some hiring managers and recruiters simply ignore cover letters. The key to that statement is that SOME ignore cover letters, not all, so why would you take the risk of being perceived as anything less than a true pro?
Case in point: Not long ago, I developed a resume and cover letter for a client. I encouraged him to call the recruiter directly, and she actually picked up the phone. Using insights that my client gained through their conversation about the hiring company's needs and wants, I was able to weave together a cover letter that spoke directly to the company's objectives.
The letter thoroughly impressed the recruiter, and the hiring company's interviewer told my client that he chose to interview him over other candidates mainly because he submitted a cover letter when others had skipped that step.
The interview went great, and my client started his new job in less than a week.
Bonus tip: Think ahead to your next most important letter by learning how to Send a Killer Interview Followup Letter.