To answer your question, let me highlight firstly that they serve different purposes. In the context of ISO 17025:2017, however, the answer would be the Quality Policy as it provides a framework to establish the quality objectives, meaning quality-focused results to be achieved.
It is also mandatory for laboratories to have policies and objectives that are aligned to address impartiality, competence and consistent operation – all quality components. The Quality Policy is the core of the management system, binding the laboratory in a singular vision of what Quality means to your organisation. It focuses your attention to structure processes and documentation appropriately, as it is the crucial focal point for addressing risks and opportunities.
A Quality Manual on the other hand is not mandatory, although useful to “bring” all your processes and procures together. The mandatory documentation requirements could be met with a master index of documents and separate procedures.