Bit of both!! KPI's etc in CV but clearly address your suitability for the role in the letter! Any experience that is relevant put on but make sure you don't overlook any mentoring/training/relevant stuff in your current role that is applicable. Generally my experience is people often assume that the interviewers etc 'know' they have experience and can do the appropriate tasks etc but you have to state it! Start by drafting your letter and CV then really be harsh and rework etc - may take some time but it will help you with the interview prep too!