For a computer science student, I would recommend Linux, as it requires you to learn a lot about how the operating system works. You will see many principles and things being applied there from your theoretical courses about operating systems, algorithms and patterns, etc. Depending on your level of masochistness, you might use a user-system, like Mint or Elementary OS, or you might want to use a super-user system, like Arch, or you might want to use a "I am Linus Torvalds" system, like Gentoo or Stali.
At some Universities, students might prefer BSD over Linux. BSD is really nice, do not underestimate it just because it is not as famous. In terms of stuff you can learn, BSD is pretty compareable to Linux.
In case you are required to have one of the above mentioned operating systems, I would rather go for the one which you have more experience with and which your professors prefer (as they will have to teach you stuff and probably do not want to debug applications for other OSs or search for alternative software)