Which undergraduate degree will prepare me better for the MCAT exam?
Biomedical Science.
The actual answer is neither major will prepare a undergrad for the MCAT exam.
Both biology and biomedical science will give a student most of the necessary foundational science knowledge needed to be successful the MCAT, but actual preparation for MCAT is on the student him/herself. (HINT–the MCAT tests far more than just knowledge. Just knowing the material is not enough to score well on the exam. You have have to be able to comprehend, analyze & evaluate novel material in variety of physical & social sciences and develop original solutions to the problems arising out of the information–all on the fly and under stressful timed conditions.)
Pre-med students can major in any field so long as they complete the required pre-requisites for med school. Just doing well in the pre-reqs is enough to allow a student to well on the MCAT so as long as the student puts the effort into preparing for the exam. Both of my daughters went to med school, neither was a bio or biomed science major. Neither were about half their classmates.
P.S. If you want to know which undergrad major scores best on the MCAT–the answer is a near tie between mathematics/statistics majors and philosophy (humanities) majors.
See: https://www.aamc.org/download/321496/data/factstablea17.pdf