CircuitPython is a fork of MicroPython, and they periodically bring in commits from MicroPython to stay up to date.
One advantage of CircuitPython is that your board gets mounted as a volume like a USB drive, and you can just copy code.py onto it to install your program; it's just a more user-friendly experience. For comparison MicroPython only exposes a file system through a tool called rshell[1], that you need to use to copy code over. CircuitPython also seems to have a lot more libraries due to Adafruit's investment in this ecosystem, although I suspect that most of them could easily be adapted to work with both.