Over the last 40 years Nikon and Canon have switched many times for which company led customer satisfaction levels of products and/or service.
Throughout the years the best means of ensuring quality service is dealing with a local dealer and building up a great relation with them, or joining the respective company's professional service.
It also helps to develop a relationship with an authorized repair center for your equipment. There are many more of these than manufacture repair facilities and often they can handle repairs in house and quicker. Or if repair center is required to send the item to the manufacturer, the repair center can do QA checks when received from the manufacturer - which can possibly save the customer trials and tribulations if an item is not repaired correctly.