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.