Basically, Science and Biomedical science students take same courses in first year.
In second year it's mostly same except electives. In third, year you pick specific modules you want to go into EX: Bio, medical sciences, Pharmacology, Physiology, Pathology, Etc.
Different modules require different average to get in.
But, its up to you what you apply for first year.
Western has two science departments called Faculty of Science and Faculty of Medical Science in Schulich school of Medicine and Dentistry. From Faculty of Science, you receive Bachelors of Science (BsC) and from medical science department you receive bachelor of medical sciences (BmSC) upon graduation.
So.. It's really upto you if you apply as biology for first year or biomedical sciences because everything is same for first year. If i have to pick, I would recommend biomedical science. It requires higher average to get accepted into.