I have always advised students to always requests for a reference letter from every job, project, class, and even volunteer work that they have been involved with.
The reason is that a reference letter is an invaluable job-hunting tool.
Because every job-hunter will always blow his/her own trumpet. Everyone will sing their own praises and nobody is going to say bad things about themselves when they apply for jobs.
In marketing terms, this is called “puffery.”
With a reference letter, however, you are allowing other professionals (higher level people whom you have worked with or have supervised you) to say good things about you and verify your suitability in the job that you are applying for.
So, which is more powerful?
I rest my case.
Still, while requesting for a reference letter, know that there is a difference between a reference letter and a certificate of service.
A certificate of service from a company is a document that shows that you have worked in the particular company. It usually contains relevant details like period of service, position, job description, and last drawn pay. You can request for this regardless of your performance.
A reference letter, on the other hand, contains an assessment and judgement of your character, personality, skills and abilities exhibited during your service at the company. Clearly, this letter should only be requested if you performed well.