Academically, there is virtually no difference between GR & EL. You will take the same classes, do the same assignments, etc. There is no difference as far as the curriculum is concerned between these two campuses.
Whether one campus is better for research than the other, that is somewhat less clear and is dependent on your interests. Anything primary-care related (including psych) will have plenty of opportunities at either campus, as both campuses have home residencies in primary care specialties. One major difference would probably be in ortho research, as GR is the location of CHM's home ortho residency, and is the place that matches CHM ortho applicants most consistently. EL is still, as far as I know, home to the Office of Medical Education Research & Development (OMERAD) so epidemiological & medical education research is potentially easier to get involved with in EL. The School of Public Health is located in Flint, so EL will be closest to that if you're interested in biostats, disparities research, etc. EL is also much closer to Ann Arbor, and I know several classmates interested in small, highly competitive specialties who pursued research through UofM.
Something to note, Spectrum just merged with Beaumont creating a new hospital system called Corewell Health or something, so being in GR will probably make networking easier within this system if you are interested in residency in Western Michigan & the outer suburbs of Detroit. Sparrow Hospital in EL just got bought by the University of Michigan Health System, so that might open doors into the vast UofM research system that was otherwise accessed simply by cold-emailing researchers in Ann Arbor.
As someone who did M1 & M2 in EL, and as someone who just completed the match process and matched into a T10 in my specialty, I can confidently say that being in EL vs. GR had no impact on the quality of my medical education and did not put me at a disadvantage/advantage compared to students in GR. Because CHM is not specifically a research-focused medical school, the onus is on you to find opportunities throughout M1-M3 that align with your goals. If you know / anticipate that you will be applying to a competitive specialty, then it is best to start reaching out to researchers / residents in a program within Michigan / follow up with your undergrad research labs / etc. if you know that research will be an important component of your residency application. Some of my classmates reached out to UofM, some reached out to residents at Spectrum/Sparrow, some started their own projects, and some continued to work with their undergrad labs.