Should I Get an Arduino or a Raspberry Pi?

They are both great platforms. The Pi can in theory do everything an Arduino can while also allowing access to a full Linux based OS (or RISC, but I can't comment on that). If you were limited to just one, I'd say to get the Pi just because you can do more with it. The Pi is also better if you're more interested in the software side of things.

In my opinion the Arduino is a bit more mature overall and has better tutorials for beginners starting out in electronics. You can also use it with Windows or OSX if Linux isn't your thing. Even if your long term goal is to do electronics on the Raspberry Pi, I'd recommend getting an Arduino to learn on because it is simpler and doesn't require Linux knowledge.

OpenCV on PI2 is achievable. You ain't gonna get there with Arduino Amtel microprocessors.

PI2 is a full environment and wee little quad-core + GPU power house and not too bad for programming especially with wiringPI, but Arduino is simpler and much more practical to learn stuff on. You can get kids in primary school doing Arduino which is not really going to happen easily with PI.

Both are pretty cheap, but Arduino is cheaper.

Get a BeagleBone Black. ;-)

I prefer Arduino, but may be just cause i do very basic things :)