If you search under Thread for BS/MD 2021 thread, someone knows more about SLU med scholars program. I think @babySimba1. Think has a sibling who went to the program and matriculated into med school. You may want to private message to find more.
Heard the program has a high attrition in the first year or two and whoever survives it has good chances of matriculation. So if you are sufficiently confident about your high school rigor and college preparedness you can go for it.
Regarding the chances of research and ECs opportunities BU is a much better place. It is also rigorous so may be more difficult to get equivalent gpa since you will be competing with many top notch class/batch mates. Their overall acceptance rate last year was 18.5.
Yes BU has a preference for their undergrads and anyone else associated in anyway. They fill about 1/3 of their incoming med class every year with those associated with BU in someway, SMED, undergrad, EAP/MMEDIC, graduate students, alumni. Not known how many of these are undergrad.
BU is far more reputed (and may be expensive in your case), and has many well known areas of study. In case if for some reason one decides medicine is not for them, easier to find another field of study of excellence at BU and switch paths and gears. Not sure if that will be the case at SLU.