If present, we always follow the ECAM. If time is available we reference the QRH after complying with the ECAM. Referencing the QRH means reading additional information which may help in understanding the anomaly. In the presence of an ECAM any action directed by the QRH is ignored.
The ECAM is the most direct method of correcting or ameliorating the anomaly.
That said there are approximately 80 checklists in our QRH which don’t have a corresponding ECAM (unannunciated checklists in our vernacular) Since no ECAM is present we reference and apply QRH procedure. Fuel Leak, Air Temperature to Hot or To Cold, to name a few.