Students routinely choose referees by seniority. Committees read for specificity. A head of department who taught you in a hall of two hundred can say very little that carries weight; a supervisor who assessed your project can say a great deal.
Choose the person who has evidence about you, then make it easy for them to use it.
What a strong letter contains
The relationship and its duration. Specific work the referee observed, with detail. A comparison that means something, such as standing within a cohort. Evidence of the qualities the programme actually selects for. And a clear recommendation rather than a polite summary.
Anything that could be written about any student weakens the letter by taking up space.
The brief you should give
Send your referee the programme name and link, the deadline, your CV, the statement of purpose, a reminder of the specific work you did with them including dates and outcomes, and a short note on the two or three qualities that matter for this application.
This is not putting words in their mouth. It is giving a busy person the material to write specifically rather than generically.
Consistency and timing
The letter should not contradict your CV or statement on dates, roles, or grades. Check them against each other before submission.
Ask early. References are the part of an application you do not control, and a scholarship deadline missed because a referee travelled is a genuinely common loss.