Hi, I saw your post and I wanted to give you insight into both summer classes and research over the summer. My freshman year, I did both. Summer classes are interesting to say the least. It's a mix of students who are retaking classes and those, that like you are trying to get ahead in them. This means that you can have an incredibly mixed bag in terms of what a class will shape up to be. The time frame in summer is often much more compressed, I don't know if where you were planning on going does this, but at many places the summer courses are only five or six weeks long (in schools with summer I/II) or twelve (whole summer courses). This means that the pace is substantially quicker oftentimes. It is easy to fall behind if you miss a class, because you may cover the equivalent to a week's worth of material during the semester. There usually is no scholarship aid available during the summer, if that is an issue. The fact that people generally only take one or two classes, however does make it substantially easier to focus on what is being presented, which in my opinion is the best thing about summer classes. There are generally 2 labs a week which are a time consuming aspect depending on what style of lab reports your professor require.
Research is a mixed bag to be honest. In three months you likely are not going to have enough time to really do that much. I had a summer with a great PI, he was incredible and even listed me as a co-PI on two impressive grants, which was nice. However the lab sucked, I was uninterested in the area of research. You really need to be a match with the area for it to be worthwhile. It was not something I enjoyed. I did synthesis work in a chem lab and loved it.
In the end there isn't an easy way to choose which is the better choice, it's incredibly personal. The research looks good on your CV, but a few extra points on the MCAT do too. I'd say take the shot at research, you can bring an org and physics book and learn it over the summer if you really want.
