I would trust a psychologist's diagnosis more than that of a Psychiatrist, based purely on their job description. A psychologist's job is to listen and offer advice. For their advice to work they have to listen to the patient and develop some working model of the patient's mind, what motivates them, how to help get things in order.
A psychiatrist can simply listen for the first five minutes, and then start you on a medication they think will work. If that doesn't, there's probably a list in their office of other medications to try. I honestly believe a lot of psychiatrists tune out their patients after the first five minutes because they've already decided which medication to try first, whereas a psychologist doesn't have that option.
Basically, a psychologist is going to have a lot more information from which to make their assumption, which in some ways makes them more qualified to diagnose. There's still the subject of psychiatrists who specialize in autism, as they're going to be specially trained to spot this stuff, but in usual cases, I trust the psychologist more.