Few days ago I bought the Arduino Yun. Arduino Yun is the first Arduino board that includes a Linux distro (Linino based on openWRT) and can be compared with Raspberry Pi.
Great news, but… it is better than a Raspberry Pi?
Quick answer: yes and no.
I think that Arduino Yun vs Raspberry Pi is not about ‘better’ or ‘worse’, it’s about ‘appropiate’. Maybe Arduino Yun is more appropiate for IoT Hardware related projects and Raspberry Pi is more powerful and flexible, allowing to setup a wide range of software (including a full operative Linux system like Raspbian or Fedora) , but indeed, more complex to use, specially on the Hardware side.
Conculsion: buy both and have fun. You love them all!
You can take a look to both hardware specs: (sooooo different!)