Both are open-source electronics platforms, and in prototyping process we can obtain more or less the same results, but they have significant differences.
With the Arduino® card you need to program in a special language, very similar to C++. Can be conected by USB to a computer to transfer the program. It provides a lot of combinations with the servos, actuators and all the components, so for us this is translated like more freedom for design.
On the other hand we had the LittleBits® system. Much easyer and simpler to make it work. We avoid completely the programming step and we can focus on the action. With very intuitive magnet connectors is easy to build an electric system, with all the components. These caracteristics make them perfect for early prototypes but not for final prototypes or robots.