They're apples and oranges, and it completely depends what you're doing. RPi can be a personal computer, Arduino is most certainly not.
When you're asking yourself whether you should use a RPi or an Arduino:
RPi Cons: RPi has analogue in pins, limited PWM etc, an OS and file system you need to take into consideration when building a project.
RPi Pros: You can use pretty much whatever language (natively) you want. It's a computer. Much better support for Audio / Video built-in.
I meant to say here 'no Arduino like analog in pins'...
Well, they are two very different things. One is basically a computer, the other a microcontroller.
edit - I know they're different. But for many projects, it just doesn't matter. Stop telling me they're different.