1. No, there is no difference in where you interview. You are not pre-assigned to either campus - you select preferences in Spring. There is no difference in curriculum at either school, they are just different feels mostly. East Lansing is on a large university campus, so it will feel much like how it did in undergrad. This comes with all the university research. There is probably more basic sciences research at East Lansing because of this, i.e. there is not an entire department for microbiology in Grand Rapids. Then again, Grand Rapids has the Van Andel institute and more hospitals than East Lansing. The hospital system for Grand Rapids is larger and supports more clinical research. This means you're likely to have more choices for this type of research in Grand Rapids. However, probably only the administration really knows how the two stack up, because the differences are imperceptible in this department, and most important, they are inconsequential. You will be fine wherever you go, and in all reality you're not likely to even want to do research. Really, the major difference is facilities and location. Grand Rapids is newer with more natural lighting. The campus is a part of a complex of medical buildings, so when you go to school it feels like you're going to work, rather than your undergrad class. Some people like that separation from undergrad, it feels more "adult". Grand Rapids is also closer to downtown, so it has a much more urban feel. Everything is really close and mostly within walking distance.
2. There is no method to being assigned an interview day in one campus or the other. They just switch off between campuses for interviews, and they assign you whichever one isn't yet full that is next.