University prelim programs are actually kind of difficult to get now because so many anesthesia/radiology programs are starting to eat up their spots with categorical programs - most university programs I looked at had between 2 and 5 spots and typically saved them for their own students. Community programs on the other hand can have 20 or 30 prelim spots a year so your odds are much better there.
I don't think being md/phd would make much difference as to which you do but it will likely make you more competitive overall.