I assume you're asking about MCAT vs GMAT?
MCAT is the Medicine admission test
GMAT is the business admission test
First, let's address difficulty from an admissions perspective. Both of these tests are scored on a perfect curve, meaning, the difficulty of the content is irrelevant regarding how high you are likely to score. What matters is your proficiency with the material COMPARED to other test takers.
So which group is more fierce? Medical students or business students?
I'd say that the competition to get into medical school is tougher than that for getting into business school. So, I'd call the MCAT tougher. Put another way, a person of equal academic ability would find it harder to score highly on the MCAT than the GMAT.
Next, let's look at experienced toughness:
There is no contest here. The MCAT is a much worse experience as a test taker. The MCAT is currently designed to be 7 hours and 30 minutes long. The GMAT is 3hrs and 7min long. The MCAT is much tougher to physically get through than the GMAT.
This depends highly on personal ability. The MCAT is what I think of as a broad test. It tests your proficiency on a super-wide knowledge basis. The MCAT lends itself to memorization. The GMAT on the other hand is what I think of as a deep test. It tests your ability to solve problems, on a much narrower set of knowledge areas. So which is tougher? That depends on your personal abilities. Personally, I find memorization tedious, problem solving engaging, so I perform better on the GMAT.
There is an underlying assumption to your question. Your question implies you might consider choosing the field of medicine or business based on the toughness of the entrance exam. Doing so would be a terrible mistake. When considering which field to enter, I would rank exam toughness as an absolute non-factor. You should be thinking about temperament, skills, interests, career factors, job roles, subject matter, etc as factors for choosing a field. After you've made your choice, then slog through the entrance exam that you're faced with. Either exam is readily surmountable with enough good practice.