In their current incarnations, I like both. Touch ID has been mostly reliable and with 2nd generation starting on 6S it has been lightning fast. I would always go into settings for Touch ID and keep adding more angles for my fingers to be read at. The fact that it allowed users to register up to 5 fingers was nice too.
Face ID is still in its infancy but it has been great for me about 95% of the time. Only gripes are when lying in bed, since it can't scan my face from an angle very well, and when lying flat on a table. I have to tap the screen and then hover over it. I'm confident that the new iPhones which adopt Face ID will see marked improvements, much like 2nd generation Touch ID starting on the 6S was a marked improvement over the 1st generation Touch ID. Regardless, I think that given the design and composition of the iPhone X, namely not having a home button, I feel like Face ID works more fluidly and seamlessly than a Touch ID would.
That being said, I am not sure how practically feasible Touch ID under the screen would be. Given all the technology that is currently under the hood and integrated into the screen, including 3D touch, I'm not sure how feasible that would be to translate.
At any rate, it appears that Apple has obviously gone all in with Face ID, and abandoned Touch ID, whether under a button or under any glass.