The Arduino platform will give you easy access a hardware/software solution. You will get a taste of c++ and be able to do simple practice programming that will do something: Blink an LED, control a motor etc. But without some sort of hardware to interact with - the Arduino won’t do much for you. With Arduino it is just you and the MCU - no operating system to get in the way. But you can do fairly sophisticated things like learn to create an c++ Object (Arduino Library) and so on.
The Raspberry Pi and the Arduino have only one thing in common: Each of them is on a fairly small board - which makes them look alike.As you know, chess and checkers are played on the same board - but completely different games.
Working with the Pi will give a taste of a Linux operating system and also let you interact with the hardware on the board. This adds another level of complexity (you have to 'talk' to, work thru Linux) in addition to working with your hardware.
Generally speaking they are both hardware intensive platforms - the Arcuino moreso. So it depends on your goals as a CS student.If you want to be an application programmer, hardware interaction can be a distraction.If you're going to work in hardware, the Internet of things, and so on, they'll be a great way to get some insight into simple, inexpensive projects.
Be sure to look at Python and the PyBoard - yet another way to practice creating projects. Also take a look at the Processing language (Processing.org) a totally graphical environment in which you can create very sophisticated applications that need no hardware - just your computer monitor. Processing is a very "c-like" environment so you can get a feel for some coding techniques.