As per previous threads on this topic, I have also used both.
My local dealer provided poor service and had no attention to detail (the car returned with more faults than it was given to them with) and therefore, my car now goes to the factory to have the annual service. Is it close? No. Does the job get done right? Yes. For me, that has a value.
Like Boshly, if I had the luxury of an RPM or someone who provided a similar quality and service close to me, I'd happily use them. As I don't, the car goes to Malvern.
I am yet to use Millers but I do hear great things about them and would definitely given them a try in the future - again, they aren't close but it seems I am required to travel if I want good competent service.