The general consensus is that admission to M.D. programs is harder;
In most situations, this is true. the number of applicants versus the number of spots is generally higher for MD spots.
and, completion of a Ph.D. is harder.
Now this, on the other hand, is just stupid.
I was going to build this whole analogy about how that was like trying to compare the difficulty of performing a face transplant or building a space shuttle from scratch. But then I realized that was wrong. It's actually more like comparing two different types of endurance races.
Medical school is like a triathlon. It's long and grueling, has well-defined stages, you know the course in advance, and you know exactly when you are going to finish.
Getting a PhD is like running indefinitely. You start running having no idea where you're going, with no idea of whether you're even getting closer to your destination. You might run for a while, only to have someone tell you that the race has even started yet and you have to go back to the start. You might run for a while only to realize you're exactly where you started and never left. There are going to be a bunch of people on your path pointing to different directions, none of which is necessarily the right way. Finally, there's going to be a guy behind you in a car. Every once in a while you can stop and ask him for directions, and show you a map. The problem is, every time you look at the map it's totally different. Plus, you actually have some opportunity to negotiate where you're actually going.
Is a PhD harder? Maybe. More people who start out trying to get PhD leave their programs without one. This probably has more to do with the intense pressure on medical schools to graduate their students because of the crushing debt they place on their students.
Also, it's heavily dependent on your mindset. If you're the kind of person who really can't tolerate uncertainty, the PhD is going to be much harder. If you're the kind of person who hates memorizing things, can't stand to sit down and study for five straight hours, and can't stand the hierarchy of having to do what your intern, senior resident, or attending says even when you disagree, then an MD Is going to be harder.