From my personal view summer classes have always been more fast paced than regular semester classes. When I was in college I would always try to take some of the easier general classes during the summer so I was not overwhelmed with all of the information I had to learn in a few short weeks. When I was in college these classes were generally for only four weeks with four hours per week and a lot of work outside the classroom. General classes are also more likely to be provided for in the summer since everyone has to take them. I see that you are a biology major; I would stick to taking your biology degree classes during regular semester so that you have plenty of time to take in what you are learning. So I would stick to your elective classes or non degree classes during the summer such as english, history, other required science classes, or math. Before making a decision on a class, I would talk to other students who have taken that class as well or your college advisor. It honestly all depends on the course load and the teacher.
Hope that helps!