They're about the same, but 2 major differences based on my research and personal experience:
1. The MicroPeak is more prone to erroneous data/no data after a launch. You need to be careful not to expose it to direct sunlight after it's armed and ready to launch. But even if you are, sometimes it won't register a launch. And no, I have no idea why this happens after dozens of launches.
2. The MicroPeak is smaller than the FireFly; the former fits in most (many?) BT-20 plastic nose cones while the latter only fits in BT-20 body tubes.
If I flew in competitions, I would not personally use the MicroPeak. I don't compete and I like flying BT-20 rockets with nose cone payload bays, so I use the MicroPeak.