(1) have you already been offered the internship? if not, you don't really have a decision to make yet, and I would suggest going to the interview and seeing what happens, and then making a decision if need be.
(2) if you receive and take the internship, you won't have time to do research as well (since internships are typically full-time, and research typically goes on during business hours as well (for the most part)). if there's evening classes available, you could potentially take those, but it might be tough.
you could always do the internship and then do research during the school year part-time (though it's a more difficult to make as much progress with research goals if you're only working 10 hrs/week vs. 40 hrs/week on a project)
(3) as someone working in the pharma industry who's also performed research in an academic setting, i think it's been valuable to me to see both sides of things. I like industry for working directly with products that are going to end users (vs. long-term research goals that may or may not pan out into anything that people will ever see) & for seeing things performed on a very large & broad scale. I like academia for working more directly with science, much less paperwork, feeling like i'm working on something that helps further a greater body of knowledge, and it's a bit more an idealistic setting (profitability is never mentioned as a driver for an effort, although i guess publications / 'bragging rights' may not be wholly altruistic).
(4) do what you think interests you the most and will expose you to new experiences. don't do it for the sake of potentially impressing an adcom.
personally, i am fairly certain that working in industry did not hurt me with regards to med school admissions. did it help me? i don't know, i'm not on any med school adcoms. i do feel like it diversified my experiences and helped me learn about things outside of a school setting (though research and volunteer experiences also helped with those aspects).
