Becoming a doctor is a long road of jumping through hoops. Ultimately, the last step is what matters most - residency. In high school, we took a variety of tests (SATs, APs, IBs, etc.) to get us into college. Then in college, we take the MCATs to get us into medical school. But the most important test will be the boards. The boards and our 1st year of clinical rotations is what determines the residency we get and thereby much of our future. I would not agree that the MCAT is a good indicator of how intelligent of a doctor you will be. In fact, most doctors do not remember any of their physics or organic chemistry. If you really had to base a doctor's intelligence off of a written test (which is faulty in itself), you would base it off of the boards score. Based off of my experience, I have seen many doctors who graduated from unheard of schools around the world. Yet if they can land a good US residency, they're set. For instance, at the cardiology lab that I work in, 8 out of the 10 doctors graduated from medical schools outside of the US. But they're doing awesome right now.
For those who want it bad enough, you can become a doctor through various means. And if you really wanted to judge a doctor at face value, you would have to look at where he did his residency and how well he scored in his boards and rotation grades.
Lastly, on the DO/MD thing. Unfortunately, there is a lot of bias against the DOs. That's just how it is in most parts of America. While scientifically, the 2 learn pretty much the same things, their approach is different, and there is OMM. Personally, I find the DOs interesting as a result, and they prove to be very useful for missions work. HOWEVER, unfortunately the stigma still stands. If you are a doctor walking down the hallway of a Californian hospital with a badge that says "<name>, DO", a common person will not know what you are. At my own hospital, nurses look down upon the DO, dubbing them "not doctors," despite their abilities. To choose DO, you have to truly love OMM and the DO approach, as well as be able to overcome the prejudices you will have to face.
If I offend anyone, I'm sorry, but I'm just trying to put in my two cents based off of a lot of experience and a lot of good advisors.