What is the 3rd player right now? AFAIK it's PIC or Atmel. I guess there are the various ARM boards but those tend to be more powerful than little 8 bit things.
They advertise Arduino compatibility, so I'm assuming that means compatibility with the AVR instruction set, but I could be wrong about that.
But that's just Arduino, i.e. high level compatibility. You have a setup() and a loop() and the same way to do I/O, but not the same underlying instruction set.
The chips on these guys are regular ARM cortex uCs.
They might be just barely fast enough to do real-time emulation of the AVR INSNs though ;)
Other major competitors are NXP/Freescale, ST, TI, and Infineon.