That's a decision your going to have to make largely on your own. I am not sure exactly why summer school would be harder for you than a regular semester so I can't really give you much advice regarding this. If you've never taken a summer class before I can give you a rundown on my comparisions of summer school as opposed to a regular full-time semester. Here it is,
Summer School:
1. Longer class hours, usually everyday M-F, which can be unbearable in terms of attention span. Especially for the harder courses like Organic Chemistry, Physics, Biochemistry etc. It was always hard for me to pay attention after several hours of being couped up in the class. However, for the non-sciences the courses are typically not as long and the material isn't as hard to follow so this was never a problem for me in these courses.
2. If your taking a science class it can be tough to keep up with the class, I took physics in the summer and we went through a chapter a day! It was extremely hard to sit in a 5 hour class and then go home and not only have to read the chapter everyday, which took me an extremely long time, but also do the assigned problems so that I was prepared for the exam - which was at the end of every week in my case and covered about 4 chapters of material that we had just gone over THAT week. I heard the organic chemistry and biochemisty sections at my school were just as bad. On the bright side the averages are typically a little lower than during the traditional semester and you can focus your attention on only 1 course as opposed to 4 or 5 during the regular semester. I still however wouldn't recommend the sciences in the summer. However for the non-sciences I would extremely recommend it, they are generally easier than they would be during the regular semester and also more fun. I was able to take 2 or 3 non-sciences per summer session, did extremely well in them, got them out of the way in a very short amount of time (4-6 weeks) and also had some time to myself.
In a nutshell, the non-science courses are extremely worth it. You can get a lot of them out of the way quickly, the professors are generally more lenient and even cut out some of the papers or presentations that you might be responsible for during the full semester and they are generally easier to get a good grade in during the summer for some reason. At least this has been my experience at 3 different universities.
For the science courses however, I wouldn't recommend it, your going to be extremely busy. Your going to be covering material that is relatively hard and in the case of the biosciences (biochem, organic upper level bio) requires some memorization. I don't know about you but when your expected to rememember detailed information in a shorter amount of time, not only is it going to make your life harder but your most likely not going to get as much out of the class. In my case, for my physics course, although I was above the low averages in the course, I just didn't get anything out of the course. I couldn't finish all the reading or the problems prior to any of the exams, and I didn't remember a thing after I finished. I retook the course a year later and found that I absolutely loved physics, I realized that I had gotten so little out of the course the first time that I didn't even ever remember covering some of the topics that were reintroduced this time around. And the topics that I did remember covering suddenly made so much sense when I had more than a day to devote to it. I feel that I would've truly missed out on this if I hadn't decided to retake it during the semester. I can only imagine that the biosciences would be the same way, I mean how much are you really going to remember if you've got to memorize things over a 1 day period as opposed to a several week period?
Anyways, hope this helps!
Edit: My bad I didn't realize you were talking about a math course, well that changes things. I would recommend math in the summer, I took 2 semesters of calc in the summer and with math it is definitely doable. The nice thing about math is that there is not really any time consuming reading that your going to be responsible for, this allows you to go to class and come home and just focus on problem solving. In addition, with math, if you are fortunate enough to devote all of your time to it - such as in summer school, then it's very easy. During the semester you'll find that you get side tracked by other subjects and when you finally get back around to a math course some of the things that you had learned are now forgotten and since math generally builds on itself this can really end up being a disaster. In the summer you'll knock everything out sequentially while everything is fresh in your head so this works out very well. You'll still be very busy but it's definitely doable and worth it.
