Apple's forthcoming FaceID technology will support just one face registered per device, at least initially. This has been reported by multiple people who spoke to Apple at this week's iPhone event, and confirmed separately by TechCrunch, but it's worth noting because it could change some user behavior regarding the device.
Face ID supporting just one face makes a lot of sense on the face of things (sorry), since unlike with Touch ID, users have only one face to use for their unlocking purposes. Touch ID ostensibly allows multiple fingers to be registered because you have quite a few digits of your own, and depending on how you're using the device or in which pocket you keep it, one might be more convenient to use than another.
The issue is that people also often register the fingerprints of their significant others, or even kids, depending on how they use their device. The phone can store up to five fingerprints, which makes it relatively easy to keep a few of your own as well as those of a significant other on your device just in case.
A one phone, one face policy obviously won't support that kind of arrangement: If you want to give your loved one access, you'll need to smile for the camera, or share a backup passcode so they can get in that way. The passcode option is only marginally more inconvenient than a registered Touch ID fingerprint, of course, but it still makes a difference, especially if someone's only very occasionally accessing your device and might not have committed a pin to muscle memory as a result.
I think this is more of an issue if Apple ends up moving Face ID across its product line, to devices like iPads that are much more often shared gadgets. Still, depending on how your use your phone, it might be another decided factor in whether you go with iPhone 8 or iPhone X – or, of course, neither.