Go for Linux based OS and play with the source code during your CS.
PC and Mac are for users, but you do not want to become a user but a programmer, so Linux based OS are best as they are open source and you can learn a lot.
Analogy:
- a TV user - knows how to use the buttons on the outside of the TV
- a TV manufacturer - places those electronics components there on the board
- a TV designer - knows why those components are there
To become a good programmer, you must [MUST] know how and why of the codes that you will write.