Both the Pi Cap and the Touch Board feature twelve capacitive sensing electrodes and plenty of code examples and tutorials to get you started. However, there are some basic differences between the two that are important to understand.
The Touch Board is a microcontroller board which is compatible with the Arduino programming platform. It functions as a touch-sensitive MP3 player out of the box and can be reprogrammed with one of our many code examples to have different functionality, including as a USB MIDI, serial or keyboard accessory that can be plugged into another computer.
The Pi Cap is an addon board for the Raspberry Pi - it cannot be used on its own. The Raspberry Pi is a single board computer which runs Raspbian, an operating system much like Windows or Mac OS, but based on Linux. This allows the Raspberry Pi to multi-task and makes internet connectivity, Bluetooth and video projects easier. It can also play back more than one MP3 (or other sound file) at a time - it is polyphonic (which the Touch Board is not).