I think you can break it down a few ways:
- Speed - Face ID is definitely slower than the 2nd gen Touch ID, which was scary fast. But it’s a little faster than the 1st gen Touch ID sensor, so it’s not bad. The fact that it starts the scan even before you swipe up to unlock does help hide the slower speed a bit.
- Accuracy - I’d say the two are roughly on par here under ideal conditions. Touch ID may be a hair more accurate, but the difference is not great enough that I care. However, having to hold the phone in a certain way for Face ID is a significant limitation that can lead to many more failed attempts than with Touch ID.
- Ergomomics - This is the one area where Touch ID is clearly superior. Face ID only works if you are holding the phone at the correct angle, in the correct orientation, the correct distance from your face, and actively looking at it. Compare that to Touch ID, where I could unlock my phone on a desk without picking it up or even while it was upside down in my pocket if I wanted to. You can argue that these differences don’t matter that much (I don’t think they do). But I think it’s hard to make the case that Face ID isn’t a step back in this area.
- Fringe Scenarios - Each technology has certain “fringe” use cases where it breaks down. The obvious ones for Touch ID are when your hands are dirty or when you are wearing gloves. For Face ID, it won’t work when you’re wearing a mask or scarf over your face. But it also has trouble with extreme lighting or when you are holding the phone at an odd angle. I’ve all but given up trying to use Face ID when I’m laying in bed because it misses around 75% of the time. I use my phone in bed quite a bit more often than I wear gloves so I find Face ID’s limitations to be more annoying. How you see that difference probably depends on the climate where you live though.
Overall, I’d give the edge to Touch ID. But, the difference isn’t big enough that I wouldn’t use a Face ID phone because of it. On the flip side though, Face ID is not enough to keep me on the iPhone X if something else I like better comes along. I’d probably jump to an SE 2 if one ever materializes even if it still has Touch ID.