There is no significant difference. With a 3.80 I would not recommend to retake a class. It would be expected that you get a higher grade and a class anyway. Once you reach a certain level/gpa it is pointless (in my honest opinion) to retake a class and I think you are at that point. You should be proud of a 3.8. But I fear that admissions committees would see it as you are simply chasing the A, unlike those with lower gpa's who should retake a class or two. If it is not lower than a C, I wouldn't retake a class. Even then, one C on your transcript will not keep you out of med school by any means. I would focus on your mcat have you taken it yet?