Depends on what you mean by harder... The bredth of knowledge for the USMLE is many, many times larger than for the MCAT. Plus for the MCAT, much of the imformation you need is provided in the question/passage.
USMLE is even more of a knowledge test than the MCAT so in this way, for some it would be harder and for others easier. You can prepare more for the USMLE so for those who struggled with the MCAT because much you couldn't prepare for many parts, the USMLE may be easier.
If you are asking which is harder to do very well on, think about the cohorts. Most who take the MCAT are never going to cut the mustard and get into medical school but everyone who takes the USMLE is already in medical school. So the cohort is much stronger for the USMLE and therefore, getting above the mean is harder on the USMLE.