Forgot to mention that psychiatrists can also practice psychotherapy (usually psycho-dynamic but increasingly cognitive-behavioural these days) as well as medical treatments which means that there is significant overlap with psychologists/psychotherapists except that the latter groups cannot prescribe....unless of course you're a nurse-therapist who has done one of the 'nurse-prescribing' courses which are becoming more common these days.
ps: the easy answer is that psychiatry pays more (90K at least for a consultant) but that takes a hell of a lot of student debt to get there and it's not easy, seriously not easy!