Phone call. You always miss exceptions in surveys, even if you allow open answers for each question, you sometimes want more elaboration to an answer. 20 people

Phone call. You always miss exceptions in surveys, even if you allow open answers for each question, you sometimes want more elaboration to an answer. 20 people is also very doable to call. Sending a survey will also result in people forgetting/less willing to answer your questions. Calling is also more personal.