Firefly 8S.
Firefly advantages:
-Slightly better build quality
-Loud beep when buttons is pressed and to indicate start/stop recording (this is a big plus for me, since I use the camera on a helmet so often and the beeps most cams make are too quiet to hear through the waterproof case)
-Better menus and user interface
-Better stabilization (max 1080p60 on both)
-Body is smaller (fits GoPro mounts on quadcopter; Thieye does not)
-Smoother and easier to press buttons on waterproof case
ThiEYE advantages:
-Price
-Better mic and audio
-Micro USB charging instead of mini USB
They have very similar video quality overall.
I've only have the two cameras for a little bit so I'm sure there's more similarities and differences. I have done a
stabilization comparison which can be found here between the two cameras. I will be doing a full comparison eventually. Been busy, though.