In the end, it probably depends on how you define "difficult," and yes, it's ultimately likely to be a futile exercise to compare the two. If I were to say anything, it'd be that the difficulty of getting into a funded psych doctoral program is comparable to that of med school; the admission rates are typically lower, but there are also perhaps a larger proportion of under-qualified/non-competitive applicants given the large number of undergrads who major in psychology.
And yes, the medical school world has done a much better job of regulating the quality of its programs than has psychology/the APA.