C1 came first, is available now and has a fully open GPU whereas the Pi still relies on Broadcomm's mostly proprietary chip.
C1 also allows you to add NAND directly to the board where the Pi is still limited to MicroSD. C1 also confirmed to not have the same bus limitation that plagues the first Pi an is unconfirmed to exist in Pi2.
C1 also has a built in RTC and a 5v Barrel Pin jack so you don't have to rely on MicroUSB for power.
The C1 has a ARM Mali GPU, which is just as close as the rest of the GPUs available on ARM SoCs. The RPi GPU is the most open in that market in that it has some documentation and source code available.
