There are lots of factors to consider when choosing a course. Which classes am I interested in? Will this course help me with my research? Will this course help me get the job I want? These are all important questions to ask, but they are not the only things to consider when choosing a course.
As was mentioned in our discussion of the course enrollment process, Cornell does not require you to choose your courses until several weeks into the semester. You can take advantage of this opportunity to attend several classes to see the professor's teaching style and find out the content and structure of the class. These can't be looked up in a course catalog, of course, and they play a big role in how much you get out of the course. You'll want to decide quickly which classes you'll actually take, though, or you'll end up trying to do homework in seven classes, and you'll be behind in all your classes before you've even decided which ones to take.
If you haven't yet decided on a chair for your special committee, then the person who is teaching the course may be just as important as the subject being taught. Taking classes is often a good way to get to know faculty and let them get to know you.
Each Cornell course is assigned a particular number of credits. The number of credits assigned to a course is generally equal to the number of hours spent in lecture each week. It does not include the number of hours required for homework and study. This number is often estimated at 2-3 times the number of lecture hours for undergraduate courses; for some graduate courses, however, it can be much higher. It is always a good idea to ask other graduate students about specific courses you are considering; they can give you an idea of the workload as well as information on course content and teaching style.
For M.S. and Ph.D. students, 3 courses of 3-4 credits each is considered a workload that will keep you busy. If you are also a teaching assistant for the semester, you may have almost no time left for research. On the other hand, there are exceptional cases in which students have taken 12-13 credits of course-work along with TA responsibilities. Before finalizing your courses, it is a good idea to discuss your time commitments with your committee chair (if you have chosen one) so that both of you know how much time you will be expected to spend on research.
Note that many graduate fields want their M.S. and Ph.D. students to register for additional credits of ``Thesis Research" to bring their total credits to 12-15 each semester. You should check with the chairperson of your committee as to whether they would like you to register for Letter or S/U grade for research credits. Various fields have different policies about it and different faculty members may have their own styles.
If you are in the M.Eng. program you are required to take 30 credits of technical classes over two semesters, but that includes 4 to 8 credits of work on your M.Eng. project. So, M.Eng. students are probably looking at 11-13 credits of non-project coursework and 2-4 credits of work on a project each semester. If you are extending your M.Eng. degree work into the summer, then your schedule may be more flexible.
If you have selected a chairperson for your special committee, then it is very important that you discuss the courses you will take with that person. If you are in the M.S./Ph.D. program, then the special committee is usually the ultimate authority on which courses you must take in order to graduate. If you haven't selected a chairperson yet, then talk to the Director of Graduate Studies in your department and consider meeting with several professors that you are interested in working with to ask for course selection advise.