When I started on my next robotics learning experience I automatically assumed Arduino was the bets and easiest way to go. The Arduino is the largest board in the pictures. The Arduino was indeed easy to program. Their IDE and language is simple. You can plug a USB cable straight into it and go. However, when I went to connect a dual DC motor driver to it (The smallest board in the pictures) I surprised at how many pins had to be devoted to it. It was a big mess and that was only half of what had to be hooked up. My other immediate concern was that there is a size constraint for the sumobot im building. the Arduino was quite bulky and difficult to design a 10cm by 10cm constraint. So I looked at some alternatives.
The alternative is in this pic as the one with wires attached. This board is known as the Baby Orangutan and can be found at Pololu.com This little board is the equivalent of the other 2 combined without compromise. It actually gives you more IO pins because the dual motor driver integration is more efficient. The funny looking solder strips next to it are my power rails, getting regulated 5v power through the board. This is good for all of the low current sensors that need to be integrated. I love how compact the solution ended up being compared to my arduino solution.
This picture shows how the strip board lines up my power rails next to the pins so that I have a nice clean way to plug everything in.
The one downside of the controller is that you cannot plug a USB cable straight into it. You have to get a special cable to program it with. This cable can be used with all ATMEGA chips without being formatted for Arduino. If you decide not to put the Arduino software on it you need to use something like AVRStudio to program it. In the end I think the extra little learning curve for the AVR IDE + the separate programmer were minor detractors compared to the benefits if they were the same price. As it stands though the Arduino + motor driver is about $50, the Baby Orangutan + programmer = $40.