In a nutshell:
MBS is good for students who need additional preparation for the MCAT and/or pre-requisite science courses.
IMS is better suited for student who have a decent MCAT but a sub-par GPA, and require no further preparation in the pre-requisite courses or the MCAT.
That being said, some MBS students stick around for a second year which confers a masters degree, but is essentially the same coursework as the IMS curriculum.