It depends on your choice, if you are starting from the scratch go for a certificate course as it will build up your foundation. A certificate course trains you in a specific area and helps you obtain necessary skills without years of schooling. It is generally completed under a year.

On the other hand, diploma is achieved after completing classes and hands on experience in a specific field. It usually takes longer than the certificate course. And doing a diploma course after a certificate course helps you strengthen your foundation.