Having used face unlock, it's really gimmicky. I turned that off right away. It could be a good idea in the future but, like with Samsung's hands-free gestures, right now I'd have to spend too much time aligning my face and waiting and hoping that it was going to work.
Touch ID takes what you would have done anyways (thumb on home button to wake and then type in PIN) and makes it even faster. Now you still put your thumb on and eliminate the typing pin part.
I haven't used Touch ID yet but I wake my phone a lot during the day, so this seems like a nice (but not absolutely necessary) feature for me. Something small that makes the experience that much better.
I use face unlock on my Nexus 7. Not gimmicky at all for a device I pick up maybe 3 times a week, in good light.
For a phone, it's a poor fit. Variance in light levels is too high.
Fingerprint login is something I've had on my laptop for what seems like a decade. It's a gimmick there. But I anticipate that fingerprint unlock could be about as good as face unlock. It has the advantage of working in the dark.
I have face unlock turned on, and so apparently, no, the natural way of picking it up doesn't perfectly center my mug.
Net effect is having waited however long to try to center and recognize my face, I end up having to realize it didn't work, so an exception case action is needed, and type in a pin and enter.
Touch ID takes what you would have done anyways (thumb on home button to wake and then type in PIN) and makes it even faster. Now you still put your thumb on and eliminate the typing pin part.
I haven't used Touch ID yet but I wake my phone a lot during the day, so this seems like a nice (but not absolutely necessary) feature for me. Something small that makes the experience that much better.