As having owned all the iPhones, the LG G3, G4, all of the Notes and Galaxies, and the Nexus 5X and 6P, here is my take.
If the phone is 5" or smaller such as the 5X, GS7, 6S, 7, etc, it is easier to have the fingerprint scanner on the front, IMHO. While the larger phones, such as the iPhone plus models, or the Note 7, or the Nexus 6P, the fingerprint scanner on the back would work best. Being that the phone is much larger, having the scanner 2/3rd's of the way up on the back of the phone and in the middle makes my index finger land almost perfectly on it and I don't feel like my thumb is straining to reach the scanner on the front.
I honestly love the scanner on the back, at least when it comes to the larger phones. Also love the 6P's method of simply resting your finger on the scanner to unlock the phone. Android/Huawei gets this right. On screen navigation buttons or home buttons on the front, and static fingerprint scanner on the back. One thing I would love to see implemented, however, is being able to use the scanner on the back as a sort of trackball or scrolling device. Simply brush your finger over the scanner, up and down, to scroll webpages and the like.
If Apple wants to clean up those bezels, it would serve them well to put TouchID on the back and an onscreen homebutton with a low opacity on the front or a capacitive style small homebutton on the bottom bezel ala Samsung phones.
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TouchID on the front is perfect imo. Advantages include: being able to unlock your phone when it's lying flat on a desk, multiple finger prints can be registered and used comfortably, makes more ergonomic sense.
I guess my last point is very subjective. Since Apple Pay is two quick taps of the home button,it makes more sense for it to be on the front then on the back. The touchid/home button are dual purpose so it's a much smarter design as well. The only reason google is doing it on the back is to obviously avoid patent infringement and to not make a 100% iPhone copy. The pixel phones really are about 90% a straight up iPhone 6 copy.
One other thing, they went with a design that has 40% of the back glass to incorporate the finger print scanner. Yuck. And good luck when you drop your phone and have to worry about not only shattering your screen, but your finger print scanner as well.
I don't think Apple has a thing to worry about this generation of phones. Notice how during the keynotes Google and Samsung almost have to mention Apple in some way or another(whether to mention why they think their design is better, or to poke fun), Apple does not mention any other competitor during theirs. Apple knows they are the leader in the industry, and they clearly arent concerned with the copy cats.
What's that saying? Copying is a sincere form of flattery. Makes sense.
The 5X and 6P don't have glass backs around their scanners. I'm not sure why there is glass on the back of the new Pixel phones, but I assure you, your "assumption" of needing it for the fingerprint scanner is wrong.
You say the only reason for putting it on the back is to avoid patent infringement, well I guess I'll let Samsung, Motonova/Lerola, and others know to watch out for Apple and their patent lawsuits.
I'll give you the Pixel phones on the sides and fronts look way too much like the 6 design. Typical HTC.
I also assure you Apple has copied plenty in it's mobile tech world and after 6 to 7 years of smartphones being around I would have believed that no one would have their blinders on this much.
Oh and about that Apple Pay. With the Nexus 6P, you virtually just have to rest your finger on the scanner which wakes and unlocks the phone and hold it up to the pay station to pay. So again, less steps/processes.
I get it, you're an Apple fan, and good for you. I appreciate the competition and all things tech as therefore I don't come off as a fanboi of any camp and am blinded by truly better experiences or ideas that one may have over the other.