I think UHS is the better choice too, and not only because I am a student there.
AZCOM hasn't graduated a class yet, so you have no idea how the students do overall (First part of the boards is not a good indicator;I love the discussions about passing rates on the boards, that all the schools try to capitalize on, but all of them have first time passing rates in the 90s% so they don't mean much to me) and what kind of standing it has among residency programs. UHS has been around for a long time and its graduates consistently are accepted into competitive osteopathic and allopathic residency programs.
AZCOM may be a very promising school, but I would rather go for the certainty than the promise, particularly when you will be investing so much into your medical education, in time, effort and money.