@bodangles is 100% correct. Two wildly different fields. Moreover, what do you mean “harder”? The question is so vague as to be meaningless.
Do you mean which is more difficult academically? Again, apples and oranges. And do you mean at the undergraduate level, or beyond? At the undergraduate level, like @EdwardKenway said, “premed” is not a major. It’s an intent and describes a set of classes you must take to be eligible for medical school admission. If you’re a premed who is not an engineering major, then engineering is more academically difficult in terms of sheer workload. This is not to say that a premed non-engineer undergraduate has it easy, but engineers have a very rigorous and standardized curriculum. It’s routine for engineering majors to take 20+ credits a semester–far more than any other major. Engineers and premeds both tend to have commitments outside of class time–in the case of premeds, this generally takes the form of research, volunteering, clinical experience, etc. and in the case of engineers, co-ops/internships and projects, and in some cases, research. Engineers tend to have a more project-heavy college experience than non-engineers, which tend to take up more time than the average non-engineer premed’s activities. Of course, if you’re a premed who’s majoring in engineering, things will get very busy.
Do you mean which is academically more difficult in terms of the material? Depends entirely on your strengths and weaknesses. Undergraduate premed requirements tend to involve more memorization than an engineering major’s classes (though this varies from course to course, obviously). Medical school itself is notorious for the sheer volume of material to be digested and is very heavy on memorization. Engineering is all applied math. If you’re poor at synthesizing large volumes of information but good at math, then medicine might be more difficult. If you’re poor at math but good at memorizing lots of information, engineering might be more difficult. If you’re poor at both, then both will be difficult. Also keep in mind that engineering majors tend to have lower GPAs than any other major, which is due to the difficulty of the coursework.
Do you mean in terms of the physical and psychological toll the education takes upon the student? Almost certainly medicine. The long hours, particularly as a resident, and the stressful nature of the work, not to mention the length of the training period (which can be considerably longer for some specialties than others) and relatively low pay during said training period, almost surely take a greater physical/psychological toll than anything most engineers go through.
Do you mean in terms of getting a job? As long as you get a US MD or DO, you are virtually guaranteed to find a job in medicine somewhere (in the US). However, the unemployment rate is also extremely low among engineers.