Historically neither of them made any microcontrollers. Arduino shipped Atmel and Raspberry Pi chips. Adafruit has boards with a variety of microcontrollers from various brands on them.
This is different now that Arduino is Qualcomm-owned and ships Qualcomm silicon, of course.
> Chief microcontroller rival Adafruit
Implying that Adafruit makes their own competing microcontrollers, which sure would have been news to me.
Edit: sees sibling posted at the same time. Well that would explain it.