Honestly? The bright blue light. The Mophie light is so dim that it can barely be seen. The Samsung one is pretty crazy in a dark room.
That, and I had been concerned about it not showing green as a full charge prior to all of us figuring out that it was probably something specific to Samsung phones. I think I paid 38.00 for the Samsung and I dunno, 60 whatever for the Mophie. Perhaps the Samsung was the better choice. I LOVED that Face ID worked with it as you were saying. I could tap the screen look at it and boom. That was immensely valuable.
Damn you. Now I might have to keep it and return the Mophie.