I have played with both. It really depends what you are looking to do with them. I am leaning towards the Pi for my custom pin. It has less IO and but will simplify my display by letting me drive a monitor. There are some options to extend the IO, I have even seen designs that use the Pi and Arduino together. Both are going to require a custom board or shield to drive any solenoids. I have some notes but haven't begun prototyping yet. I am trying to get the Mechanical design and practice my software programming first.