I recently came across the Quora question, Which one is harder, engineering or medical?. I figured that being an engineer myself and knowing quite a few people in the medical field, I was in a pretty good position to answer. So, I relatively quickly hammered out a short answer, which has since turned out to be by far my most viewed Quora answer. It's a little surprising considering how little work this answer was compared to other things I've written for Quora, but I guess that's the way it goes. Anyway, below is a copy of what I wrote.
I'm an engineer. My wife is an RN, and through her, we have several friends who are MDs. I've even gone along on a few medical missions and witnessed surgeries first hand. And I would say that you can't make a blanket statement that one is harder than the other. They're both diverse fields, with more and less challenging paths in each.
For example, as an engineer, you could earn your bachelors degree, then go off to a manufacturing company in a well established industry, and do nothing but look up values in tables and plug in numbers in already developed formulas. That's not very challenging at all. Or, you could earn a PhD, go off to a research institution, and try to solve new and fundamental problems in your field (e.g. Advanced Rotorcraft Technology - Research). Medicine ranges from family practice to epidemiology to pathology to surgery to countless other fields.
I do think it's more stressful / difficult to actually become a medical doctor than an engineer. MDs have to go to graduate school, pass their licensing test, and complete their residency (almost like an apprenticeship). Engineers simply need a bachelor's degree. Granted, engineers can earn PhDs, and can do a lot of on the job training and continuing education throughout their careers, and can do the EIT to PE path (our own version of an apprenticeship, which is more important in some fields than others), but all that's not required to simply become an engineer.
So, it depends an awful lot on the specific field of engineering and medicine. There's probably a higher minimum level of competency among MDs than engineers because of the more difficult path to become an MD, but at the more challenging levels, I think they're comparable. After all, the two go-to phrases to emphasize intelligence are 'rocket science' and 'brain surgery'.